Re:
1. The definition of IMC and what SVFR means varies between types of aircraft AND the Class of airspace. Also, cloudbase has nothing to do with whether you are IMC in a helicopter in Class A airspace. The limits to permitted visibility for SVFR in helicopters varies depending upon what you and others are doing and where (from 1 km to 10 km if I recall correctly). However, the cloudbase en-route is never a limit to SVFR.
If you are IMC in Class A airspace you must be IFR, but you could be if you had the IR and a suitable helicopter, as ST suggests.
2. I am not quite sure that I understand 2. ATC gives you a clearance, If you follow it you are OK. If the conditions prevent you from continuing in a accordance with the clearance, you tell ATC and a plan is organised to sort it out. Lots of hele twins using the London Zone SVFR can accept an IFR clearance, although it may not get them where they wanted to go (e.g. no IFR approach to Battersea).