Well, tbh, I know next to nothing about the repetiteur scholarship. I've never personally known anyone who have applied, let alone getting an offer. But I think the competition would be fierce as st catz is the only college that offers this (at least according to the website!) and therefore the number of places available must be very limited. You'll probably have to excel at piano to get an offer.
As to the choral thing, from my own experience, it is not difficult to get an offer even if you haven't been properly trained for singing. You have grade 8s in two instruments, that is amazing! And your sight reading must be good. The choral audition is ALL about sightsinging. If you are singing in your school choir, and you're confident with your sightsinging, you will have a good shot applying for this. The audition takes place on the 20th or around of Semptember and you will know whether they will be offering you or not immediately afterwards, which will leave you plenty of time to make up your mind before submitting your UCAS application. The choral foundation colleges, i.e. Christ Church, Madgalen and New are generally the most sought after and therefore the most difficult ones, and in the second group are Queen's, Exeter, Merton and Worcester, which have got a better reputation than others and a bit more difficult to get in than other average colleges.
As to qualifications like diplomas or other things, they simply do not matter! There are not many applicants each year, so they can audition all those who have applied. And when it comes to an audition, it's your performance that matters! They want people who could do real things, rather than simply showing off their qualifications. If you wish to have a go, just do it, and there's nothing to loose.