wow that is horrible ;( I am so sorry but i feel your pain as my dex is from a pet store but luckily other than pancreatitis has been pretty good. My backyard breeder dog dee dee has been the worst
ok so it sounds like your one has kennel cough
which is contagious
and Giardia and coccidia are also contagious and i believe the ear mites are as well. The first thought was ear mites when you said ear itchy and cannot believe the vets did not catch that as those are pretty easy to catch
also i never pluck the ears as it opens the pores and the only two times my boy had them done he got ear infections so will not do that.
Usually they use metronidazole i believe for the giardia - there are alot of sharp people in dealing with giardia and coccidia - thank goodness have not dealt with any of above but i did deal with ear mites and they treated both my dogs as my dd came from breeder with them.
Unfortunately you as i have learned a very valuable lesson here and it is not fun trust me. Hopefully this will be the last of health issues but unfortunately i have been dealing with them for years with mine - It sounds like this last vet is good though and cathcing everything. Clavamox is usually used for uti infections not the above.
You are going to have your hands full for a little bit but it sounds like you are great people and you will get a ton of help here to get these boys back on track. Hang in there
I recommend wee wee pads and when they poop toss out as i do not think you want them pooping in yard and reinfecting each other but others may better assist with this