Hi OP.
There are two issues here. Firstly, your Cambridge offer. You told them in your application that you already have a Maths AS, and that you will be sitting Maths A-level in June. Cambridge made you an offer based on the strength of your application. Well done; meet the terms of your offer, and you'll be a student there in October
Secondly, your Maths A-level. There does not seem to have been a mistake with your AS entry last summer - P1 is compulsory, and you were given a choice of P2, M1 or S1. You took P2, and passed the AS. Again, there is no issue here.
CIE has two routes to get an A-level: (1) the staged route, where you take P1 and either M1 or S1 at the end of the first year (you also get an AS if you follow this route), and (2) the option of taking all four exams together at the end of the second year.
You are taking route (2). Effectively, then, you will have two CIE Maths qualifications (in addition to your Further Maths A-level): you have already completed your AS, and you are about to sit your A-level.
To pass your A-level, forget about the exams you took last year; they've been used up for your AS. You will have 4 exams this year:
P1 (exactly the same course you took and passed in 2018),
P3 (a subset of which you took and passed in your P2 exam in 2018), and
either (a) a combination of Mechanics and Statistics - M1 and S1,
or (b) just Mechanics - M1 and M2,
or (c) just Statistics - S1 and S2.
So, as far as I can see you do not need to tell your Cambridge college anything. You told them you'd be doing A-level Maths this summer, and that's exactly what you'll be doing.
I realise that ideally you should have been on the staged route (1), where you would have done M1 last year, but that's water under the bridge now. You studied for the rest of the pure mathematics stuff last year, and have concentrated on the applied work this year.
If I have read the issue correctly, I do not see what your problem is, unless you have not yet studied the work for one of your optional modules?