So, let’s take it from the top:
=> nonnonononononono. Cambridge is NEVER required to give anybody an interview, much less an offer! The required qualifications (achieved or predicted) are a minimum to get to the interview- one that most successful applicants will have exceeded in some way or another. The test scores, together with your Personal Statement and Rec are evaluated by the tutors (professors) in the subject that you are applying to- not an AdCom. The tutors decide who they want to shortlist for interview (fyi, Cambridge interviews a higher percentage of applicants than Oxford does, but a lower offer rate after interview, and more people who don’t meet their conditions). After the interviews the tutors discuss who they want to make offers to.
There is a special form of torture that we haven’t mentioned yet, called the Winter Pool, where applicants who are considered good enough to get in but whose original college decides not to take are placed. During winter pool colleges with places left to fill go through the files of the people in the pool and pull out ones that they would like to offer a place (that’s Cambridge; Oxford pools before interview).
The main point is, your AP scores are necessary but not sufficient.
=> Yes, you could be given an offer conditional on your Senior year APs, and if you miss it by even one you will lose your place. Once you have your offers from all your UK universities you ‘firm’ your first choice, and if it is a conditional you are allowed to ‘insure’ a second choice, for a uni where you either have an unconditional offer or the conditions are ones that you are confident you can meet. US students can do this, and/or have a safety in the US, on which they pay the deposit in May, and withdraw in July when they get their scores. And yes, it is not wonderful to be the only one sitting an AP exam whose marks actually matter. Also, fyi, Cambridge is notoriously firm about meeting your conditions.
=> Cambridge is a bit coy about GPAs- they want a ‘high pass’. My hunch is that if everything else is right, a 3.5 would not be a deal breaker (just an opinion, obviously). Note that most UK unis really don’t care even that much- and the ones that do are like St Andrews, which actively recruits Americans.
=> Be aware that psych, stats, enviro and comp sci are not considered full APs- they count as 1/2 APs. Calc BC, Physics and Bio would put you in a fair position (though as you can see from their website they would prefer you to have Chem as well). Your other APs together would add up to 5.
One thing to remember is that they want to be sure that the students that they accept are strong enough to succeed in the program. As somebody else posted, you are effectively coming in as a sophomore- and they expect you to hit the ground running (seriously- it is not unheard of to have work due by your first class meeting of first year). It is hard to overstate just how intense the workload at Oxbridge is. If getting the scores you need on the APs seems too stressful and too much work, you might want to consider whether that pace and intensity is something that will suit you.
Even if you think it would suit you, if you are really serious about science and the focused curriculum you should look beyond Cambridge (would anybody really only apply to Harvard?). Imperial, UCL, Bristol, Durham are all strong performers in your field, and all of them have less stringent requirements than Oxbridge. Durham is even collegiate (like Oxford & Cambridge). With UCAS you can apply to 5 courses for 1 fee, with 1 essay and 1 recommendation, so once you have done one you just add the others onto the form.
