the domain in which you are specializing is fascinating and some rudimentary knowledge of it is useful, even indispensable, for art historians and for other Sanskritic śāstras including kāvya -- should hence evoke a more than lukewarm interest of Indologists and readers of this list...
It is indeed "the nub" of my argument to take "sahita asya to represent sahitaḥ asya rather than sahite asya" because the latter option, as we have seen, leads nowhere except for turning an author elsewhere eager to communicate meaning accessible to 99 percent of his contemporaneous public, into an obscurantist who can only be fathomed by 1 percent
-- plus a whole tribe of philologists centuries later who are ready to accept suddenly, ad hoc, a Vedic sandhi, etc., in what otherwise seems to be a śāstric text in impeccable classical Sanskrit.
Next, trying to think in line with your argument and in the wider context of your interpretation, what could be the syntax of the verse?
Apparently there are two options separated by vā. The expression yadgṛhe asks for a corresponding term, in the first option tatra, in the second option, I would suggest, tena (gṛhena, rather than, in your interpretation, janmalagnapatinā).
Both options lead to the same result: asya ... labdhis. If tatra and tena go with yadgṛhe, asya probably does not refer to yadgṛhe -- here I would modify my previous suggestion: could it go with janmalagnapatir? Could the janmalagnapatir, the lord of the first house, have something to do with the own body and hence with aṅgasukham?
Then, if both options lead to the same result, what could be the precise difference between them? In the first, the janmalagnapati is said to be uttamavīrya, and the house where he resides is (positively) aspected
-- whether tatra ... dṛṣṭe is a locative absolute or whether it is in direct concord to yadgṛhe. In the second, in my suggested reading, he is not necessarily uttamavīrya, but at least sahita with "that", i.e. with that house. MW 1095 col. 1 gives "(in astron.) in conjunction with (instr. or comp.)" for sahita. In western astrology, "in conjunction with" is always a matter of plus or minus 2-3 degrees, so that a planet can be in conjunction with another planet or even a house even if it is not squarely coinciding or residing in it, even if it is just outside that house: possibly Indian astrology is here more black and white,