And just for a bit of reassurance, I've used xeno
a lot over the years because of my set up and have never had any problems... I'd go so far as to say it's quite routine here. I have between 2 and 4 piggies at any given time and take in older pigs, adult 'last' pigs and rescue pigs. So we might get a new arrival and they might be carrying passengers so we get prescribed the xeno routinely (whether mites are positively confirmed or not, because mites are pretty common) and everyone is treated - 3 times.
And the shedding and scratching stops, and we've never had any problems - we love it, very effective.
Vets have different attitudes to this stuff. There are 2 strengths for different size animals... because you can use ivermectin in a lot of different species for all sorts of things. We get xeno-450 which is for animals over 1.2kilo. There is xeno mini-50 for smaller animals. Not everypig here has been over 1.2 kilo, some have been 1.1 or even 1ish, but they can work out the proportion of the tube to spot-on. We don't count the drops - I need glasses and I'm not that accurate - but for smaller pigs they give a narrow syringe to suck up the contents and do a bit of maths. But sometimes a very careful vet might use the xeno mini-50 for what is essentially an adult size pig and that's where things can start to get expensive and messy because you need more than one tube to get the required dose for bodyweight, and all your pigs should be treated together at the same time whether they're symptomatic or not because they're contagious. As you mentioned counting drops I'll assume you got the 450 and your pigs are adult size(?)
You must do the repeat doses and they should be two weeks apart. This is because xeno does not kill the eggs and so they will hatch out and start to grow to breeding maturity. This takes about 3 weeks, so you do the repeat dose at 2 weeks to kill them before they start to lay again. The 3rd dose two weeks later is to mop up any stragglers. Conveniently a box of 6 xeno-450 tubes is good to treat a pair of porky piggies for the full course! (and fyi I once got accidently prescribed 2 doses 3 weeks apart instead of 3 doses 2 weeks apart and of course the mites came straight back. An easy mistake for that young vet to make - but he was embarrassed enough that he won't make it again!)
The mites spasm as they die so treating a heavy infestation can lead to a couple of hours of distressing itching for piggy before things calm down. Mange mites are horrible and the itching can drive them to harm themselves trying to scratch. You'll know after treatment if you got it right because the scratching pretty much stops! The stuff wears off after a few days so there will be no lasting effects. I dob it on the bald patches behind ears, where I see the skin in rosettes, or by lifting the hair along the spine and dobbing it at the roots. It needs to soak into the skin, not the fur.
If you see no difference after treatment in their itching, and they've had the correct dose for bodyweight, fungal infections are also pretty common. I don't know if I've ever heard of piggies being allergic to wood shavings but I guess you could try different bedding if you thought that might be an issue... but I suppose if it was then most of the scabby patches would be on the tummy area where there is contact. Let us know how you get on x