The word "compromise" is going to have to come in here as well.
Honestly, you can't just lay down the law to your folks (and extended family) about all they are doing WRONG. Because they'll likely tell you he's lucky they're taking "care" of him at all! (and they likely are right).
You also can't educate them if you present them with this overwhelming "list" of all they're doing WRONG. Instead, part of it is you have to find things they are doing RIGHT.
No dog ever died because it didn't get exercised *enough*. However -- a dog that is left to its own devices is going to be harder to deal with ... he's going to be bored and will likely find things to get into. He's also going to be more of a handful because apparently this one is 'fearful' -- and essentially you may have to be captain of re-training because that takes a TON of effort and effort that your family may just not be able to make.
First off -- make a list of those things that literally could just plain be FATAL if he got hold of them. Things like Tylenol/acetaminaphen, anti-freeze (which means they can't let him just 'run' in the driveway or garage as well as monitoring him when he's loose) -- those things are just plain FATAL to dogs. full stop.
However -- you might want to boil it down to what he CAN have as far as allergies. Does he need to be wheat free? Make them a list of suggestions of things he CAN have -- like I give mine Gerber Graduates (freeze dried bananas, strawberries, yogurt plops and apple bits) -- the dogs LOVE them and people love giving goodies to the dog.
In other words it might be easier just to tell them Misha can't have wheat (or whatever it is) but here is a whole BAG of stuff that IS ok. How about fresh fruits and/or veggies -- tell them what to expect, but it's something they likely HAVE already and I don't know any grandma in the world who doesn't want to give the "grandkid" something yummy AND good for them.
Many times, said loved ones "sneak" things assuming it doesn't hurt and frankly no amount of guilt is going to get thru to them. But if you give them easy "legal" things suddenly they can really LIKE the idea of giving the dog some 'real' thing that is good for it.
How about talking to Gram about making liver treats for him ... if she has an EZ recipe she may go with it. but trying to guilt her out about things he shouldn't have? Usually with most families it just ain't gonna happen!! AND it makes you sound more like a 'brat' than you really want to be.
It's scarey how fast we can go from concerned guardian to "brat" when we're trying to get the family to get with the program. BUT once you get your foot in the door that some of this stuff IS do-able and "HERE IT -- IS SO IT IS EASY FOR YOU" then suddenly they'll cooperate.
Likewise with exercise. Rather than saying "Misha HAS to get MORE exercise" ... try "You know, it really bothers me that I can't be home mid-week to take him to the dog park and I really feel badly about it because I know you guys really don't have time. But gosh he LOVES it so much and you should have seen him today -- he did REALLY well!!"
And just leave it at that. If they say "well I don't even know where this park is" then you can tell them about the 15 foot leash (cos he's really too much of a fraidy cat dog to go without a leash and that keeps him under control). But if they don't -- the honestly, it can just wait til you get home.
I went back to school for THREE YEARS back when Prissy was a young dog (she lived to be 21 incidentally -- but had major HUGE bouts of pancreatitis several times during her life) and my folks kept her while I was gone.
Did she get enough exercise? No.
Did she get what *I* wanted her to eat? NOPE (and that was a biggie because of the pancreatitis -- but after my mother fed her the wrong thing and she got sick WHILE MOM HAD HER suddenly it was a whole different story).
Did she live thru it? Yes. And I was able to keep it 'cool' with my parents as well -- it was the longest 3 years of my life. At one time it was over 18 months between times I saw her -- but she survived and so did I.
None of us are perfect and it's hard when parents have to take on responsibility that THEY perceive is not theirs (and even if Misha is a 'family' dog they aren't going to automatically fall in line with what *you* deem is best for Misha!!). So work from a positive angle with them too.
You do likely know far better than they do what Misha needs ... but you can wage that battle only to LOSE it and lose the whole war in between. If you make taking care of MIsha into too big of a huge hassle, then you don't want them to just give up entirely, right??
I'm not trying to step all over you -- but helping family "learn" how to care for the dog can be dicey.