Sookeh, it sounds like you are doing your very, very best for the boys and I'm so sorry you've had such an awful time with this, especially when you're not feeling that well. Hopefully, the cuddles and wheeks from the boars will help compensate a little bit!
With regard to probiotics, ironically, Pets at Home sell Vet Ark Pro C, but obviously wouldn't recommend getting it there. This place also sells it www.thehayexperts.co.uk/Health+%252526+Hygiene.12/ProC+Probiotic.351.html and there might be a few others on line if you google it. They might do next day delivery if you're struggling to get it from the vets. The vet does a slightly more expensive, perhaps stronger one, but the one in the link is the one I give to mine when they are poorly.
One of the many things that annoyed me about the vet hospital I took one of my boars to in February with cystitis is that while they seemed fixated on putting vitamin C in the water (which actually isn't a good idea as it can react specifically with plastic bottles, potentially leaching out harmful chemicals into the water, and if pigs have plenty of vitamin C rich foods like yellow pepper and broccoli, they don't need supplements,) but never once mentioned probiotics despite the heavy dose of antibiotics they were giving him. When I asked about probiotics they were like, "oh yeah, whatever." 
70 is right about mites - they can have a few without problems, but if they get ill or stressed, mites can thrive and cause them problems. There's info here on mites, but it sounds like your vet is clued up on this www.guinealynx.info/mites.html.
It was 10 or 12 years ago that I had a pig with mites, with an open sore over his shoulders that hurt him like the dickens. I believe it was Peter Gurney (I got him from Peter - RIP Peter!) or perhaps Cambridge Cavy Trust (CCT) that recommended applying Eurax eurax.co.uk/ to give some pain relief. I don't know if that would still be recommended, but it seemed to help mine.
There are alot of over the counter preparations for humans that are okay for piggies, but it's always good to check guinea lynx or the Guinea Pig Forum to make sure they are "okay." They aren't licensed though for sale to treat anything other than humans, so when buying from the chemist, you sometimes have to invent an itch of your own :) or for some things, pretend to have an elderly aunt, or similar.
Best of luck and you are doing your best, you really are.