Is it possible it is a matter of interpretation or minhag?
Yes, it is possible. Hence my wording:
It’s more likely a matter of proper dikduk.
That’s my opinion. Don’t like it? Fine, so we disagree.
(I’ve been told to say “morid ha’geshem” but I have heard “ha’gashem” with a kamatz, and I’ve seen it in siddurim.)
That one’s actually pretty easy. Is “gashem/geshem” the end of the sentence or not? If it is, then say “gashem,” otherwise, say “geshem.”
My own practice is to view it as one long sentence “[He] causes the wind to blow, the rain to fall, sustains the living with kindness….” and so I say “geshem.” If you don’t view it that way, then say “gashem.”
The Wolf