I'm pretty sure you can't sneak in D&D while standing behind an enemy.
You can, and can even hide in plain sight once good enough.
Fun > realism
Stealth during combat isn't unrealistic... Just going "I am sneaking" while being directly observed without anything intervening is unrealistic.
Again, welcome to D&D,.
Also last I checked its a magic spell that makes you literally disappear, so yes, they can disappear into thin air
