My Bosmer assassin did it at level 5, but only because I didn't think of it until earlier. It's actually not that difficult to complete that quest without killing a thing, even with OOO/FCOM. She got in and got out exactly that way... didn't even have to kill a slaughterfish. Now, she missed out on some loot, to be sure, but she can always go back another time. With a distraction potion or two, I'll bet you could do it at level 2. Then you lock problems are solved for the most part.
OK. My char is always attacked by several Trolls already in the entrance tunnel leading down. In fact, I'm often surprised by how good they are at detecting. And they are really nasty. But I guess I've been unlucky or too loud (my chars never sneak). Anyway, if it works it's a great tip, but I have only horrible memories of that cave.
EDIT: Here's how I solve the issue of opening very hard locks. Guess some might consider it exploiting, but it's all available if not nerfing it with other mods. Anyway, first my chars always live in Frostcrag Spire, so the first objective is getting it up and running, including the Spellmaking Altar. Then you must have access to the Fortify Skill effects, perhaps easiest via one of the Doomstones. And then I make a Fortify Security 100 pts on Self for 3 seconds spell (or more, depending on Restoration skill). With the spell you can open all locked containers and you also tend to disarm and get poisons from the traps. Very convenient and possible to have from Level 3 or so, but perhaps a bit cheap as said.
Fortify Acrobatics 100 pts for 5 secs is also nice - allows access to certain locations that have handplaced Ayleid Meteoric weapons for one thing.