Why mode A when squawking 7700?

[mm_flynn]

Just checking my understanding of Mode A vs Mode C.

Am I correct that

1. These are two different protocols, but use the same SSR frequencies ( a bit like AM vs FM on a given frequency)
2. All 'Modern' (in the Bookeworm defined sense) transponders will sense and respond to either a Mode A or Mode C interrogator. In addition, the response will be appropriate for the interrogator (that is even if you have Alt selected the transponder's response to a Mode A interrogation will be a Mode A response)
3. The Alt switch simply fills in or blanks the Mode C framing pulses when responding to Mode C
4. A transponder which can only reply to Mode A interrogators (i.e Bookworm's Old) wouldn't respond to TCAS (as it is a Mode C/S interrogator) but would respond to most ground stations (because they are Mode A/C interrogators)
5. we don't seem to know if TCAS would 'see' the Mode A response to a Mode A interrogator.
6. We all use a historic shorthand which is technically wrong and use 'Mode A' to refer to a modern transponder which is only outputing the 4 digit squawk and framing pulses and a 'Mode C' to refer to one outputing altitude data as well as the 4 digit code. Even though in both cases the transponder is responding to Mode C interrogations

Do most of the ground radar units in the UK still interrogate in Mode A ?