Participant.
The Rambam at the end of Perek 7 of Kiddush Hachodesh writes that the reason for Lo Adu Rosh, is because the the calculated molad is an average and often the real molad falls later. The make up for this discrepency We keep one day and push off one day.
The Raavad argues and says the reason for Lo Adu Rosh is the morecoomnly given reason to avoid Y”K o sunday/Friday and hoshana Rabbah on shabbos. This is not mentioned by the Rambam.
R’ Chaim Kanievesky in his Shekel Hakodesh explains that the Rambam doesnt disagree that those are why Ad”u were the days chosen to push off. He is just explaining why it is pushed off at all. If the calculated molad was the real molad there would never be any dechuyos, since it is an average and is not accurate which requires occasional adjustments, we push off for these other reasons. Though still not arbitrary.
I don’t understand why maintaining a 29 day month is important? and what would be the driving factor to determine whether to keep 29 days or not? if every year was always 354 days alternating 29/30 It would quickly fall away from the molad ( the average molad is not 29.5 days it is 793 chalakim longer) and even from the seasons as the 19 year cycle wouldn’t fit either