Hello
I can't answer all your questions but I can tell you what I have experienced with my DS.
He has silent reflux (at it's worst he did sick up) and is medicated. Not MPI though and he took regular formula.
From birth to now he only drinks half the guidance amount of milk. It was a constant worry to me for a long time, esp before weaning. However his weight gain has been fine.
We control his reflux with meds and a milk thickener. Gaviscon is as good as useless IMO although i was glad of it for a very short time as it did help just a little temporarily. The thickener is carob powder, here it is by Cow and Gate and called carobel I get it on prescription. It is calorie free, has zero nutritional value, harmless, doesn't constipate, and can be used in varying quantities to make just a bit thick or a LOT thick. because it is a thickener rather than a specialist formula it can be added to anything so we were able to transition from formula milk to cows milk fine and continue to use the thickener (I realise you won't be going to cow's milk, but whatever you move to when you stop the formula...I'm looking ahead there).
I did not wait to wean. I planned to start at 6 months on the dot. DS had different ideas and wanted food much earlier, I managed to hold him off until 5.5 months but in the end it was getting ridiculous, he was crying for solids, I couldn't eat anywhere near him, he was so upset and so hungry.
I also went the BLW route, DS has never had a puree. He has had a few soft foods like humus and cream cheese but no purees. In our case (and others I have read of) it was not a slow start. He was hungry and he wanted the food NOW. Babies weaned the puree route don't necessarily take to it immediately either. It isn't a race and whatever route suits you is fine. For us BLW was a huge success.
Starting solids actually increased DS's milk intake, odd as that sounds. He suddenly needed an additional milk feed following every solids meal (was also given water or milk in a sippy cup to drink during meal).
Honestly I felt like I'd been starving him since birth.
I would not recommend ever force feeding a child. If your LO is losing weight this is a matter for professional medical help, not force feeding.
Like I said, I can't answer all your questions but maybe something here helps a little.
Good luck with whatever you choose to do x