1. As
Kaioshin says, they're probably just going with the "insanely popular" Hirano Aya. But I'd have preferred to hear more of the others, too.
2. It's hard to say if Aya is that popular with moe-ota, since there is a constant undercurrent of hatred for her among some people on 2channel.
3. I'd also have preferred to hear Fukuhara Kaori straight on "Valentine Day Kiss", but Aya was hilarious behind her.
4.
AVPlaya, I think they just did them all once (really) and just cut them short for the TV ED.
Here's the interview with Aaya on the official site. They ask her about the songs and she says the director told her at the recording session that since Konata is an otaku, she should imitate the original breathing, timing and vibrato exactly. Since she had already rehearsed her songs on her own, this made things difficult for her. She expresses nervousness about the result and apologizes in advance to the "anime song world."