Let me consider the section to illustrate the use of the nyasa and how it relates to the mantra. From the manuscript of Sahib Kaul, I quote: "ṛṣyādi smaraṇaṃ vidadhīta yatha tatrādau śrīmahāgaṇapateḥ asya śrīmahāgaṇapati-mantrasya śrībrahmā-ṛṣiḥ (śirasi) gayatri-chandaḥ (mukhe) śrīmahāgaṇapatirdevatā (hṛdi) gaṃ bījaṃ (guhye) hrīṃ śakti (pādayoḥ) namaḥ kīlakaṃ (nābhau) mama caturvarga-phala-prāptyarthe gaṇapati-prasāda-sidhyarthe jape viniyogaḥ iti ṛṣyādi-nyāsaḥ [...] atha mantraḥ hrīṃ gaṃ hrīṃ gaṇapataye namaḥ" (emphasis added).

Here, it is not simply a matter of uttering the mantra. This is the pitfall of seeing ritual as simply a performance, since a performance exists only in its activity and is ontologically distinct from the performer or actor. This is not to say that mantras cannot be performances; a lay bhakta's jap of 'om gaṇapataye namaḥ' is still an acceptable use of a mantra and correctly a performative activity. But in other rituals (like Kaul's), the ritualist embodies, enacts and re-produces (re-creates) the mantra. Through the placement of the ṛṣi on his head, he conjoins his mind (manas) into union with the genius of the ṛṣi (the dṛaṣṭā), through the placement of the chanda on his mouth, he conjoins his speech (vāc) with the sanctity of the metre, and by the placement of the devatā in his heart (the āsana), he establishes the devatā within him (sāyujya). In effect, his enunciation of the mantra is not a repetition of a mere phrase composed previously. Thinking (and seeing) as the ṛṣi, speaking as the ṛṣi spoke in the chanda and with the devata present within him, he speaks forth and manifests the mantra. He (re-)creates the mantra in its most original form (and not simply as a phrase handed down through a lineage). Consider how the term 'kalpa' is both 'creation', 'ideation', as well as 'ritual' bound by an ontological connection. Thus, the ritualist is no more ontologically separate from either the ṛṣi, chanda, devatā or even the mantra. The ritualist's enunciation of 'hrīṃ gaṃ hrīṃ gaṇapataye namaḥ' is only the externally observable effect of a self-transformation that has already taken place or begun within him, and not its cause. Through this, the ritualist re-cognizes himself (cf. "so'ham") and completes the self-transformation. The entire prayoga (in the nitya-paddhati) may be seen as a series of such successive self-transformations.

If we take a more laukika example (albeit of limited comparison) to understand the two usages of the mantra, consider eating a mango. One person consumes the mango, desiring to taste its flavor and to satiate his hunger (and having done so, moves on and forgets it). Another person also consumes the mango, savoring its taste and satiating his hunger, but doing so while reflecting the eternal play of mango farming and harvesting, of the fertile grounds and the nourishing monsoons, the unending play between food and its consumer (himself). In other words, in seeing beyond the obvious (through his insight/dṛṣṭi), he allows himself a glimpse of the immortal within the mortal.

The Guptavatīṭīkā cites the Śaktisaṇgama-tantra ("ṛṣi-cchando-devatādi paṭhet stotre samāhitaḥ | yatra stotre na dṛśyet praṇava-nyāsam-ācaret") that the ṛṣi-chanda-devatā must be yoked whenever there is to be recitation and where these are not known or given, one must conjoin the praṇava in its place. For its own prayoga (in the Saptaśatyupodghāta , the Guptavatīṭīkā mandates ("ṛṣi-cchando-daivatāni śiro-mukha-hṛdi nyaset | śaktibījāni stanayostattvāni hṛdaye punaḥ hrīṃ caṇḍikāyai vyastena sarveṇa ca ṣaḍaṇgakau") for example that the ṛṣi-chanda-devatā must be conjoined at head-mouth-heart and (specific to their tradition) the śakti to the breasts and the tattva to the heart. This varies by ritualist tradition and generalizations are unlikely to yield reliable conclusions. I would say that the reason they may not be easily found in textual format is less due to secrecy and more because (a) most of it is so well-known in nitya praxis that codification is superfluous and (b) too much variation (and scope for innovation) resists the production of standardized codes.

As for your question regarding the kīlaka, you are correct that it refers to a bolt or pin. It refers to the inaccessibility or unusability of a mantra in prayoga / ritual application. So, in a way, both the answers you have received hitherto (namely, that is a bolt to be taken out to release the mantra, and that it is a bolt to restrain the mantra lest it injures the speaker) are actually correct. Some mantras are said to be dangerous so they have 'utkīlana' rites. Not all mantras (or even tāntrika mantras for that matter) have an associated kīlaka; most simply have the triad of ṛṣi-chanda-devatā. The śakti-bījā-kīlaka goes one step further. Again, while the exact way these are understood and employed vary by tradition, so my account here must not be taken for a generalization. Some mantras have greater applicability (they can induce certain effects in the world by the ritualist's will) but these are not easily accessed or employed by those who know their words or syllables alone. One would need the appropriate adhikāra (eligibility / aptitude) - whether by initiation, family, bestowed grace, etc. - to know the skilful ways of employing the mantra to yield its power. Thus the caryā (the lifestyle/practices that must accompany the ritual - nitya observances, etc.) is often what 'seals' the mantra from general access (even if they are read in a text or overheard somewhere); its meaning is constituted and preserved more in lived experience than semantics. The Kīlakastava that is part of the Caṇḍī tradition speaks of "itthaṃ rūpeṇa kīlena mahādevena kīlitam" (8b) and "niṣkīlañca tathā kṛtvā paṭhitavyaṃ samāhitaiḥ" (16b).

Put another way with the previous metaphor of the mango, a third way is to access the mango tree itself and to grow as many mangoes as one needs. Or perhaps think of the milk and the cow. The milk is easily attainable though soon spoilt. The cow is difficult to keep and maintain, but provides a supply of milk. Similarly, the skilfully understood and employed mantra becomes a vessel for desires fulfilled (kāmadugha). The Guptavatīṭīkā, citing the Ḍāmara-kalpa, writes: "mantrāṇaṃ pallavo vāso mantrāṇaṃ praṇavaḥ śiraḥ | śiraḥ pallava-saṃyukto mantraḥ kāmadugho bhavet namontaḥ śāntike puṣṭau praṇīpāte ca kīrtitaḥ | vatsākarṣaṇa-homeṣu svāhāntaḥ siddhidāyakaḥ | yantrabhañjanakāryeṣu sughorabhayanāśane | vaṣaḍantaḥ prakalpyastu grahabādhavināśakaḥ | uccātane tu saṃprāpte mantraḥ phaṭpallavānvitaḥ | ete pallava-vāsasas-tat-tat-karmaṇi caṇḍipāṭhepi ślokāntādau yojyāḥ" - the pallavas (namaḥ, svāhā, vaṣaṭ, huṃ, phaṭ)

are like the clothes of a mantra, while the praṇava is its head, mantras become wish-fulfilling when they are conjoined with head and pallavas, and different usages are given for achieving different outcomes. Having specified this, it cites the Ḍāmara: "nyāsahīno bhavenmūko mṛtaḥ syācchirasā vinā | apallavastu nagnaḥ syātsuptaḥ syād-āsanaṃ vinā | guruṃ vinā vṛthā mantraḥ śravyajāpasty śūnyakaḥ | nirvamyo duṣṭadattaḥ syād-anyabījastu kīlitaḥ" - of which the last part is interesting because it shows a meaning of kīlaka.

Finally, the nyāsa of the kīlaka in the navel (nābhau) also carries its natal symbolism with respect to the śakti-bījā-kīlaka triad. The navel is paradoxically both an opening and an enclosure: an inaccessible access. By conjoining the kīlaka with the navel, the ritualist establishes a natal (creative/productive) link with the mantra that is thereafter sealed and therefore unalterable (as is the bond of birth). Loosely compared, the ritualist generates the mantra within himself and his navel becomes the inaccessible seal that may be taken to mean the unalterable (natal) bond now-established - i.e., the  kīlaka/navel securely holds the mantra within his body. Just like a womb (cf. śakti) envelops the seed (cf. bījā) and binds it to the natal cord, if you see the final mantra, you will see the gaṃ bījā enveloped on both sides by the hrīṃ śakti - thus creating hrīṃ gaṃ hrīṃ - and suffixed with the namaḥ kīlakam. These are elements added to a mantra - often distinct words or syllables, often not so - that evoke its latent transmutative potential. Just as the womb, seed and natal cord 'produce' a baby (which is ontologically neither womb nor seed, but a distinct entity), the ritualist manifests the mantra by transmuting its śakti-bījā-kīlaka within himself to bring forth its phala[=in the form of his kāmanā].

Thus, 'om gaṇapataye namaḥ' is a mantra too, as is 'om gaṃ gaṇapataye namaḥ', but the mantra shown here - hrīṃ gaṃ hrīṃ gaṇapataye namaḥ -  encapsulates a specific idea within a specific ritual tradition / prayoga paramparā (giving it specific meaning in the process).

I re-iterate that this is only one way of interpreting and understanding the ritual praxis. Generalizations are difficult when it comes to ritual, and what is a good way of explanation for some traditions may not be so transparent for others. Nonetheless, this kind of work is in the purview of the Prayogaśāstra Research Group at our centre under my supervision, so we would be happy to be looped in on such questions and investigations.

I hope this helps. Please forward to the Indology list if it is of relevance.

S. Bhattacharya