Can warders equip dropped armor and if so which kind (leather, chain, ???)
Yes. Just has to be tradable. In general, the best stuff you're going to get is mage-summoned stuff.
Unless they changed it, attuneable can be equipped as well, but the pet will remove it once you zone, or pocket it.
Wasn't only non visible slot attuneable disappearing from the pet? Not that it matters for me, I feed the pet every piece of defiant I get these days. Keeps them from cluttering up my bags.