appropriate vs inappropriate handling

[Bogertophis]

Hard to answer, because there are "shades" of difference between good & bad.

Head touching: done abruptly will make you no friends, & possibly get you bit, but done gently during handling, as I do with all my snakes, it's a way to
teach your snake that you are no threat to them no matter what, and the next time you have to take one to the vet or let a flaky friend hold them, you may
be very glad they had some practice. My snakes have been "bullet-proof" with public meet & greets for years, and all of them get handled near my face &
get their faces touched so that it's never all that threatening to them when it just "happens". One thing to do is let them slide thru your hands & gently
touch their face/head as they go. Or as some say, "become the ground"...snakes don't bite the ground or holes they crawl into...& they DO learn.

Opening the mouth: Not something I routinely do because you can mess up their teeth or cause abrasions that lead to infections, but over the years I've had
my share of snakes that I've successfully tube-fed to get them healthy...there's a gentle & right way to do it. If you suspect a mouth issue, you need to look.
I wish snakes would yawn more often, I always try to get a look when they do. (-why would you want to just "do this"?)

Dangling: matters a lot the physical condition of the snake...an old snake or one that's out of shape, with too much body fat, no...never. You're "asking for"
internal injury or dislocations, IMO...not that it doesn't happen in the wild, but these are our PETS that we care about & for. Some snakes are more arboreal
than others too...ever see photos of rat snakes mating while hanging from a tree branch? Arboreal snakes tend to be thinner, so if you're talking about a
heavier-bodied terrestrial species, like boas & BPs, I'd say no...avoid letting them dangle, especially when it's sudden.