Bit late to the party, but here is my two pennorth. I could have written your post. In fact, I did write your post, with added tears, about two years ago, and about my DS.
If you're sensitive and a worrier, try not to spend too much time comparing your preparations with what they do in prep schools. It's really easy to get overwhelmed by the sense that you are somehow ignorant of what you're meant to be doing, and that prep school parents all Know What To Do and You Don't. Prep school parents, I suspect, are all simmering in an environment where they're all applying to the same small number of schools, so I imagine it gets very intense. If you're absorbing all that competitive hype, but your child isn't getting the same school-based preparation, then you're going to absolutely peg out from stress.
I remain convinced that DS was judged firmly in context when up against a boy from a prep school. Must have been, because there was definitely a gap in polish and achievement. He didn't have any achievements! I would also say that he's bright but no prodigy, so perhaps intellectually similar to your DD.
I worried about his lack of extra curricular activities, but I believe the schools all took a wide interpretation of the phrase. He was practising for Grade 2 sax at the time, and I've no idea if it made any difference between offering a place to him or the next boy. However, it made us feel better as parents, which is really important too, if only so you don't transmit negativity. We forced him to get through a few books he could talk about at interview. He did a Saturday football kickabout. That was it - no language classes, no gym squad, no swimming galas, no rugby, no cricket, no tennis. Every interviewer (five schools) told us that he'd talked almost exclusively about making movies with his Lego figures on his iPod. I'm honestly not showing off - I'm not! - when I say that they all offered him a place, so they clearly didn't expect him to be doing an extra curricular activity every flipping day, or to have certificates coming out of his ears.
We didn't wind him up too much about the interview with lots of practice or nagging about eye contact, and we didn't send him to a tutor for "interview practice". You will find that Mumsnetters start posting the questions that are doing the rounds. But in my very limited experience of one boy who, far from simply ADORING learning, plods whiningly around museums like he's going to have a root canal, I can only assume that they know how to get the best out of kids, and we kept telling ourselves to trust the system. In retrospect, it was a useful mantra, though that's easy to say once it's in the past!
Mumsnet tipped me off to a couple of schools that I didn't know about, which was incredibly useful. DS had his heart set on the first school he saw... till he saw the second. Show your DD enough, and she may not be too obsessed with SPGS. Like you, we were realistic enough to know that DS was not going to get into Westminster or St Paul's.
We applied to five schools and were concerned that we were overdoing it. Going by what people seem to be saying now, that's become the norm. We applied to an ability range, from St Benedict's in Ealing to Alleyn's. (St Benedict were operating to a mandate to try to attract more baptised Catholics, which may or may not help you?)
Finally, we applied to some grammar schools in the run-up to the independents, thinking it would be good practice. He passed the initial selection hurdle to allow him to take the schools' individual entrance exams, but didn't pass any of the exams proper. It shattered everyone's confidence: his, mine, and DH's. I will never forget his little face in the bath asking me if he passed, and then, knowing he'd failed, if his friend had passed (he had). The only good thing is that it nudged us into firing our ineffective English tutor and getting a last-minute (although expensive) English tutor of inspirational brilliance who transformed the substance of his English in about eight weeks, albeit with a lot of co-work from me at home.
Sorry, really hideously long post with lots of probably redundant information that other people have already said in far fewer paragraphs. But I really wish I could make you feel better. I just wanted to emphasise that my state primary, bright, no-special-interests-or-talents-at-this-stage boy came through this wretched process. (His reports in Year 8 had best remain undisclosed, however. 
) Good luck!