------------------------------------------------------------------

-----------------------------------------------------------------

कृषीष्ट is indeed the paninian आत्मनेपदि आशीर्लिङ् form of कृ करणे. I have seen these forms appear from time to time in stotras, not elsewhere as far I recall. कृषीष्ट makes up the bulk of these occurrences. भट्टिकाव्य showcases such forms a few times too.

To add a few more references to your collection:

1. जेघ्नीयिषीष्ट (from जेघ्नीयते, the यङ् intensive of हन्). Bhamaha quotes a verse with this to demonstrate the fault of using obscure forms.
2. भविषीष्ट seen in Utpaladeva's शिवस्तोत्रावालि (आरब्धा भवदभिनुतिरमुना....अखलिमेव भविषीष्ट)
3. कृषीष्ट in the first verse of बिल्वमङ्गलस्तव (..मङ्गलं वः कृषीष्ट ॥) in the version edited by F. Wilson.
4. पुष्टां तुष्टिं कृषीष्ट स्पुटमिह भवतामट्टहासोऽष्टमूर्तेः from a stotra attributed to शङ्कर
5. Stotra by Vedanta Deshika— "अकम्पनीयान्यपनीतिभेदैरलङ्कृषीरन् हृदयं मदीयम्"

6. Panḍitaraja Jagannatha
गवेषणं ते सफलं घटिषीष्ट ।

------------------------------------------------------

-----------------------------------------------------------

Here are a few more occurrences of the particular form
that prompted your query, kṛṣīṣṭa. It seems to me not all that
extremely rare; apart from the several attestations which you cited,
it can also be found for instance in (apart from grammatical texts):

1) Bilvamaṅgalastava vs. 1 (in the numbering of Wilson in her edition of
1973); the verse ends ... maṅgalaṃ vaḥ kṛṣīṣṭa;

2) Veṅkatādhvarin's famous Viśvaguṇādarśacampū verse 39 (this is in
the section about Ayodhyā), with the main sentence of the verse being
kṛpārasaṃ … kṛṣīṣṭa sa ṛṣīṣṭakṛt kṛpaṇataikatāne mayi||;

3) the Lalitāstavaratna, vs. 19, see e-text here:
http://arunambika.ru/shri_lalita_stava_ratnam.html

As for other ātmanepada benedictive forms, note that the second
person singular form from the same root,
kṛṣīṣṭhās, occurs for instance in Bhāgavatapurāṇa 10.3.28 and
in Nārāyaṇabhaṭṭa's popular Nārāyaṇīya 11.94.6.

A few examples of forms from other roots in Buddhist texts are
discussed in an article by David Reigle,

https://www.academia.edu/6423804/The_Virtually_Unknown_Benedictive_Middle_in_Classical_Sanskrit_Two_Occurrences_in_the_Buddhist_Abhisamayalankara

More can of course be found...

--------------------------------------------

------------------------------------------------

we have mṛṣīṣṭa from mṛ by Pan{1,3,61} and laviṣīṣṭa from lū by Pan{3,4,116}.

And allusions by David Reigle on 2 occurrences in Abhisamayālaṅkāra, ........

Gaayatrii dhiimahi precative m. Whitney§837b

ved precative `fasse que je sois libéré' muk.siiya

-----------------------------------------------------

-----------------------------------------------------------

Yes, it is a benedictive ātmanepada form. See Kielhorn (A Grammar of 
the Sanskrit Language, 2nd rev. ed., Bombay 1880, § 382): smṛ, 3. sg. 
smṛṣīṣṭa or smariṣīṣṭa; saṃskṛ, saṃskṛṣīṣṭa (last example only in the 
German edition 1888).
http://archive.org/details/cu31924023200862

According to Thumb-Hauschild (Handbuch des Sanskrit, 2. Teil:
Formenlehre, 3. Aufl., Heidelberg 1959, § 563), in Classical Sanskrit
benedictive ātmanepada forms are "even rarer" than the corresponding
"rare" parasmaipada forms.

See also Renou (Grammaire Sanscrite, 2nd ed., Paris 1961, § 331: "Mais
la formation est ancienne et des précatifs de type véd. subsistent
dans les S[ūtras] (notamment Śāṅkh[āyana] Ś[rautasūtra]
Mān[avagṛhyasūtra]), en particulier au moyen" ("But the formation is
ancient and there continue to be precatives of the Vedic type in the
S[ūtras] (in particular Śāṅkh[āyana] Ś[rautasūtra]
Mān[avagṛhyasūtra]), especially in the ātmanepada").

. . . .

If Macdonell was aware of the form in the Bhāgavatapurāṇa, he may have
regarded it as a Vedicism or intended archaism. In classical Sanskrit
such forms are extremely rare, if they occur at all. For some Purāṇas
and a certain type of Stotras this may be different.

-------------------------------------------------------------------

-------------------------------------------------------------------

Renou's Grammaire §330 (p. 450) "précatif" deals with the form: "B. Au moyen (...). Le radical est au gu.na comme à l'aoriste moy. en -(i).s-, sauf que les racines ani.t en .r- présentent le degré zéro: kṛṣīṣṭa "

-----------------------------------

---------------------------------------