I don’t know about the mites I’m afraid, I’ll leave someone else to answer that!
Grass does have the same effect as hay (as hay is just dried grass), so if you have access to grass, I would keep cutting some and mixing it in with hay to tempt them. It could also be to do with the quality of hay. Often pet shop hay is quite dry and dusty, and it can be unappealing to bunnies used to eating grass! If you’re in the UK, I buy huge sacks of beautiful quality hay that is as green as grass from timothyhay.co.uk. There’s also a brand called Readigrass that make ‘pure dried grass’, which they might like. It’s too rich to make up all of their hay permanently, but it could be good for transitioning. You can also try different types of hay like orchard/meadow hay or oat hay. Most bunnies love alfalfa, but need to be weaned off it when they’re adults as it’s too fattening for an adult diet.
They’re quite young to be having veg, but if they’re used to it then you can probably carry on in small quantities. Most people advise waiting until they’re 12 weeks for veg.
Transitioning bunnies to free range is a process, and it can be overwhelming if you give them a whole house at once! it’s great that that’s your plan, but I would take it slowly, and start them off in a hutch or pen so that they know they have an area which is theirs, and so they learn where their food and litter trays etc are. Then you can gradually expand their space.
Lastly, I know you didn’t ask this, but they’re going to need to be separated i’m afraid. You can wait until they’re around 8 weeks, but at some point after 8 weeks, their hormones can kick in and cause nasty fights, even between siblings and bunnies that have up till now been living happily together. It’s also very difficult to sex young rabbits – you don’t say what genders they are, but it’s very possible you have a boy and girl who could end up mating – rabbits can get pregnant from ridiculously young ages, and I imagine the last thing you want is a litter! It’s also dangerous for mother and babies when the mother is very young.
For more info, it’s worth checking out the BUNNY INFO section, which has useful information on things like bedding,rabbit safe veg, litter training and bonding.
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