You really are going the extra mile for these two, they will pay you back one day, you just wait!
I would keep up with the hay- even if it isn't the best it is hay that provides the warmth, as you know. As far as upping their feed goes, yes, I would, but do it slowly and wait to see results- I always use beet pulp- would not feed without it, and I have never had any problems with them eating it, but I do know some have. For this reason I would suggest giving it separately to start to make sure they will eat it, that way you won't waste any feed, which you could do if you just mix it in. Get the pellets if you can, they do need to be soaked a bit more but are better value as they swell up more- I soak in boiling water for half an hour and throw away the excess water- that gets rid of the extra sugars, you don't really want those.
I would definitely worm them both- initially with Panacur Guard five day, it is very very mild and will not harm them, also, doing it over five days, you sneak up on the worms and clear them slowly, instead of killing them all dead in one go, so, again, a nice mild way to do it. Fenbendazole (the chemical in Guard) does not get all the worms, however, and you will need to worm again in a couple of weeks, with Ivermectin - any generic Ivermectin will do- and you will need to address Tapeworms as well, especially if they do not pick up, for that you will need a wormer with Praziquantel in it, DO NOT USE QUEST or any Moxidectin based wormer, it is not safe for Minis.
In the mean time, you are doing great, do not worry too much, it does take a while to cover ribs and kudos to you for noticing that the fluff was covering ribs not fat- I know of many experienced Mini owners who think their large bellied animals are "fat", in fact it is the bane of my life, so well done.
Happy Christmas!