I've never gotten around to tryin it, but I've *heard"/read here that boiling doesn't hurt them. Some of my MFs came with instructions that said *to* boild them before the first use. But I guess I'd better cover my backside by adding "YMMV".
FWIW, my washer's "sanitary" cycle gets *hot*, hot enough to, well, sanitize textiles, and it's never damaged my MFs.
On the softener issue (and I do use it on cotton towels), make sure the dispenser in the washer is cleaned out. And yeah, softener build-up in the dryer can be an issue but IME it comes more from the use of softener sheets than from softener in the wash transferring to the dryer's drum. Still wouldn't hurt to wipe it out with solvent now and then, that's what I do (with proper "don't blow yourself up!" precautions, of course).