Harry Spier wrote:

Snip..Snip..
A few more obscurities:

1) verse 18.72 of the Bhagavad Gita:
Belvalkar(1962 edition)and a few others read ...praNaSTas...
J.A.B. van Buitenen and a few others  read   ...pranaSTas...

Whitney in his grammar section 192 a. specifically mentions "pranaSTas"
as an exception to "n" going to "N" after "r".
Is Belvalkar in this edition a misprint? What does B.O.R.I. say? Is
there some optionality in this rule? Or some other reason for this
difference?
 

My copy of Belvalkar's critical edition (B.O.R.I. 1968) only gives pranaSTas, the second version and mentions the alternative as occurring in "some MSS " without any explicit reference.

I will defer to the true grammarians, but the exception is standard. I believe that the "pra" is not considered the part of the pada and hence does not invoke conversion rule n->N. There are some exceptions to the "non conversion" listed.
 

 
2)  Verse 18.43
Belvalkar (1962 edition) reads ...kSattrakarma...
J.A.B. van Buitenen reads      ...kSatrakarma...

Is this the same situation as the previous discussion on patra/pattra
and if so, is kSattra etymological or phonological?
 

Again, my copy only has kSatrakarma and the only other variant listed is a kSatrakaM karma (pardon me if I am mixing conventions, but I am more comfortable with ITRANS). There is a long list of manuscripts using this. I believe the kSattra variation belongs to the earlier discussions about the double t.
 
3) verse 15.5
Belvalkar reads    ...gacchantymUdA...
van Buitenen and all other editions at hand read ...gacchantymUdAH...
Misprint in Belvalkar?
 
This time, I am confused about the notation. It appears that the only difference is the visarga at the end and my edition does have it without any additional comments. So, it is most likely a  misprint in your edition!

About the transliteration convention, should it be gacchantyamUDhAH ? I mean should there be an "a" after the "y" and the "d" replaced by "Dh" to conform to devnagari? Pardon my ignorance about the HK notation.