1) Based on
https://crawl.develz.org/tavern/viewtopic.php?f=5&t=11099, 11 UC is exactly as effective as wielding a +0/+0 quarterstaff with 14 quarterstaff skill, which is to say that it's a decent place to put your offense before branching into other skills. 10 is slightly worse, but probably not significantly. I would generally be inclined to get Dodging to a reasonable level before going back to UC or starting Invo, but your mileage may vary.
2) Nemelex, Fedhas, Kiku, Yred and Makhleb are all very powerful (as they are for most characters). Ash and Trog are a little worse than for other characters, since you won't be wielding a weapon. Okawaru's Heroism is very strong with UC. Vehumet can allow you to branch into conjurations as a second line of offense, but both conjurations and UC are XP-greedy.
3) Chei works best when you have a full-LOS form of attack, since he makes approaching enemies very painful. Chei is far from the best choice you could make, but if you go Chei, make careful use of its invocations to manage space, and position yourself with incredible care.