So, we came up with a nasty little combo. I've talked her into trying a wizard, because I think it'd be better for her to make a few fumbles with preparing her spells rather than her choosing to learn spells she might never use. This also allows her to take Fey Mystery Initiate and add her intelligence to health instead of con, which would make up for her lower health due to being an elf. It also gives her more skill points, which will fulfill the skill monkey part of her concept.
We did, however, came up with a brilliant idea. I'm playing a DMM Persist cleric/bone knight, and I currently use it to persist three spells (Divine Power, Recitation, Mass Lesser Vigor). I do have a Fortifying Bed Roll, which means I can sleep for an hour, regain my rebuke attempts, and persist three more spells.
The idea? If she takes her first level as spellthief with master spellthief, she can steal a prepared casting of DMM: Persistent Divine Power - and gain it for the entire day. This allows her to make full attacks with a composite long bow while invisible, and with hunter's eye, gain a healthy sneak attack while doing it. I checked the spellthief excerpt online, and since I can't find anything saying they can steal ONLY arcane spells, I think this should work.
So, so far we've got: Spellthief 1/Wizard 4/Unseen Seer 4, with feats: Nymph's Kiss (bonus skills), Point Blank Shot (trade scribe scroll), Fey Mystery Initiate (3rd), Master Spellthief (6), Rapid Shot (9). With Divine Power, this means she'd get +12/12/7 to hit with her composite long bow (more with enchantment and dexterity increasing items) and, with hunter's eye, deal 1d8+6d6+4 damage. I want to work Craven into the build, but don't know if the bow feats would be more important.
So, is there any problems with this idea, or ways to improve/change it for the better? I know Spellthief isn't a base class in PHB, but the DM has made exceptions (warlock) and has created NPCs using spellthief levels, so I think it should be allowed.