alphaalpha,
Re: your para c
You are of course welcome to call for a FIS but don't assume that you will be allocated a discreet squawk and have your Mode C verified, it is quite legitimate to provide a FIS without allocating a squawk. If you want to transit CAS you will be allocated a squawk and the Mode C will be verified but if you're VFR you won't be separated from IFR traffic although you may be allocated a route or level which takes you away from the arrival or departure track.
Re: your para d
It is one of the anomolies of the airspace in which we operate that anything outside CAS is deemed separated from operations inside CAS. In the example given earlier by TC_LTN, you can legally have IFR traffic inside CAS at 3000 feet pass overhead known IFR traffic underneath CAS at 2400 feet.