You have received a lot of good advice, but when it comes down to it you need to decide which direction to take first. Here are my priorities.
1.) teeth and poop – if you mare has chewing problems, she will have digestive problems and her poop will have large pieces of undigested hay. If her poop is smooth and moist, most likely your vet was correct that her teeth are fine.
2.) worming – I am presuming that she has continued on a worming regimen since you mentioned she looked good when you bought her.
3.) stress – hotter horses do tend to show stress by dropping weight. If this is the case, she will come around. Are you seeing any other signs of stress, is she calling to her old friends, pacing or trotting fence lines? If not, her stress is most likely under control and not affecting her weight.
4.) the fact that she takes a long time to eat should not be a concern – better too slow than too fast as horses, by nature, are grazers. the concern here would be that her new friends don’t get to eat her left overs.
5.) lastly, keeping her diet the same until you transition over to what you buy locally. I have my personal ideas on the difference between high protein hay such as alfalfa vs high quality grass pasture / grass hay. The last hard keeper I had was fed minimal alfalfa hay, balance of hay was a high quality grass hay, and I am lucky enough to have year round irrigated pasture that he was turned out onto a couple of hours every day. I do not feed grain, instead he had a pound of All-in-one so his salt and vitamins had something to stick to, and a few pounds of Stable Mix. There are a zillion (or so it seems) types of weight gaining supplements you can try. I have tried a few, but have found that increasing the Stable Mix worked best. It is nice and crunchy and digests easily. If you do decide to try a supplement, my personal choice is Platinum Performance – it is a bit pricey, but with my hard keepers, it worked. My current horse is a very easy keeper, so her expenses are spent elsewhere! Supplements should get 2 – 3 months of trial to determine if they are working.
I hope this helps!