We are going through a similar problem with one of our piggies atm (it's been 5 weeks now, and is absolutely exhausting). From what you are saying I agree with TEAS that it sounds like a problem with overgrown molars (despite what your vet says). Our piggy needed to have an incisor removed and didn't start to eat properly afterwards. He saw 4 or 5 vets in Yorkshire, had a CT scan and 2D x-ray but no-one could find anything wrong. One of our vets also said she thought there was 'slackness' in the jaw, like your vet. In the end we took him to see Simon in Northampton (10 days ago). It turns out it was problems with the molars afterall (which also caused incisor problems), and all the other vets had failed to diagnose this. Pendleton was eating veg again the same day after seeing Simon, it made such a massive difference! He is due for a follow-up procedure this Saturday. We're not out of the woods yet, but Pendleton is now eating veg (green leaves mostly), grass and some pellets on his own (but still needs syringe feeding). I would recommend considering the long trip to Simon, it will be worth it.
Regarding keeping the weight up, I agree it can be a real struggle (and stressful!) We have found Emeraid to work best; Pendelton seems to tolerate it and it's good for keeping weight up. Sometimes we mix it with Oxenbox CC or baby food for variety. Also using a 1ml syringe is much easier and faster than one of those larger 15ml syringes! Hope this helps, and keep us updated!