q for quaff. This takes a little less time if the potion in question is on your shoulders or belt. There'll always be five healing potions in the starting room, along with three dimdoor potions and a see invis one. If any of those are missing, something with an inventory and a willingness to pick junk up is in the room with you, or just walked through
To gain a level, hit d, then g. Also, check
the manual. There's a lot of good stuff there. ? in game brings up a full command list, among other things.
As for the unstuck, you auto-succeed on those if you roll a twenty, which probably isn't going to save you in a combat situation. The -7 is coming from armor/shields, which negatively impacts certain skills. Heavy armor characters actually tend to be harder to play than lighter armor ones, at least in the early game.
Faerie fire is indeed an AoE debuff, and will anger your allies.
Some (well, all, technically, but it's generally not an issue unless you're using size-enlarging spells/abilities...) tunnels are smaller than others, and creatures that are over a certain size can't enter them. That's what kept your riding lizard friends from entering. Some guys are small enough to enter, but must 'crouch down' (or something to that effect)to do so, which gives some combat penalties if you're fighting in there. You could have also rode any of the riding lizards -- hit y, then select mount (It starts at H if you haven't used the 'y'use menu for anything else, but after that, it'll bring your last used command to the top.), then whatever you want to ride. You have to be beside them, of course.
Gripplis are really nice. They usually have a couple of healing potions on them, which you can either barter for (t -> b, if you have at least... 13? Either that or 10. charisma. You can also request them, even if your cha's too low to barter) or murder them and loot from the corpses. They're also
really flimsy, so you can usually one-shot them. Which is good, because it keeps them from drinking the healing potions
Not sure what killed your friend. It may have been a frost slug, which are naturally invisible but usually non-hostile (to the player, not necessarily anything else). If something you can't see starts biting you in the early game... it's probably a frost slug.