This may be outdated, I don't think I've played a thief seriously since before they even had fade, and anything here is just stuff you can figure from normal playing, I can't give any specific info about the effects or zones/items.
A lot of your problems with sneak may be because of awareness and mobs reactions to being stabbed (unless you happen to be running around in metal armor or with something humming). Some mobs are just naturally aware and will see through sneak and hide all the time, but even ones that aren't have the chance to become aware when backstabbed. Generally stabbing things that are standing up is bad and should be avoided if possible, this also goes for the initial attack. Try hitting mobs for your first attack instead and only backstabbing when they are resting (or sitting) in subsequent attacks. Some mobs are smarter about this than others. You can get used to the way it works by attacking something that doesn't have aggressive memory and seeing what sets it off to stop resting when you sneak. In some fights your best option may be to keep fleeing and attacking the mob as if you were stabbing, but just evenoming and hitting it rather than stabbing. Being patient can also help a lot if you're struggling with sneaking in on things, instead of just fading out and running back instantly, fade out and leave a room further for a few seconds, then sneak back. If you failed your original fade, or if the mob was aware, it will never rest and trying to re-enter immediately will cause it to instantly attack because it is standing and ready. If you move another room away where it can't see you, it will rest (unless it never rests- which some mobs don't) and when you re-enter, if you failed your sneak or if it went aware you will still have a second to react while it stands back up. I'm not sure anything aside from major actions breaks sneak, sitting/resting/attacking/etc. Not even sure about fade, again though finding some non aggressive memory mob is an easy way to test this stuff. Just get a horse from a stable and hit it with your hand, then fade away and come back, see if it rests, or sneak in on it and when it rests try a command you want to test, then walk back out and in, see if it gets up. If you are missing a lot of stabs on a mob that never seems to stay resting, it's most likely become aware. With sneak at v good you should be sneaking like 90% of the time, so there's something you're doing or wearing that's disrupting it.
Easiest way to level hide is to find a place where there are non aggressive mobs that scan, there are a lot of guards that do this. You'll notice them by the way they will quickly move into your room if you are one room away from them, they tend to walk out and immediately re-enter. Take something like a horse with you and hurt it (probably not in front of the guard), then take it to where there are guards and hide until the horse rests. The guards moving in and out of the room will make you learn. This will work with anything that moves around but if you can find a spot where there are scanning mobs, or just a lot of wandering non aggressive mobs it can go fast. Ungrouped players can also make this go quickly, if you do the same thing but outside of an inn or something, so long as they don't kill you. Hide can help with backstabbing things, it's a kinda situational skill though, most times it serves the same purpose as sneaking in on something but has greater risk. The best use for it was usually against other players, has somewhat limited use with mobs. Can be useful in some spots if you are waiting for mobs to separate or for something to move to a specific spot and don't want to be in combat, but if you aren't sure about whether the mobs are aware or sense life you can be in for a bad time.
Envenoming your offhand as far as I remember does nothing, I don't know if that has changed in the last few years, but envenoming your primary weapon before each attack is generally good practice, it will stack up somewhat, mobs that hit hard make it more noticeable.
Evade I think will skill from any mob casting area, problem is if you are soloing you don't want to be fighting something long enough for it to be doing that. You might be able to find some stinky rooms that can help you out though, or a weak mob that does something similar that you can fight for a while with your bare hands. Unbalance- if you don't want to group you're going to have to either find mobs that will fight eachother, or find some charmie, there's plenty of easy to get ones out there that are summoned through items but you do need to know where to look.
Pick/disable/detect are just a grind really, I would just have a routine to run through in between whatever else I was doing each time I recalled. Disable is the one that's a real pain, there used to be some extremely easy spots to skill it but the ones I knew are all long gone, now it's just a bit of a slow process unless you run a big gamut through specific places. Pick, there are some trick spots out there still, look for locked doors that have a key nearby that is re-usable and you will have it to superb in no time. Detect can go quickly if you find an area where there are lots of mobs that like to hide, and especially where they move around a lot, rather than just trying to detect traps (kender, faeries, thieves, things like that). If you can get something that hides and follows you around trying to steal, you can lead it into different rooms and detect in each room. Also when detecting traps make sure you detect the room itself and the trap itself to get an extra use of the skill. There are spots in a lot of older low/mid level zones with shady descriptions or areas that are specifically designed to help you get through this part of thieving. Some locks are totally unpickable, so it's not just you (possibly)
Case used to have to be used on ungrouped people to skill up past poor, most people never bothered because the use of the skill wasn't worth the consequences anyway. It may now learn from using it on mobs that are strong and will go aggressive but I'm not totally sure about that, I think it has some perks to being used now, maybe helping with backstab or something but I'm not actually sure about that either. Plant I have no idea if there's any use for it other than bothering people. It's technically possible for it to be used in questing things but I have no idea if any quests ever implemented it.
Nightvision on thieves is excellent, true seeing is better if you get the ranks for it.
Thieves are extremely strong solo, but they do get a lot of that strength in the last few levels and at the peak of their skills, as well as having a massive jump in strength when given the right equipment and gadgets. To play one really efficiently it takes a lot of game knowledge, but they can be very fun to play when just skilled and leveled up with ok gear. Playing one at legendary is on a whole different level, probably the most broken thing ever in the game.
