A and C
You're not quite correct in stating that the Subpart F (or 145) and G have to be seperate companies, in fact most approved compaines have both approvals. There has to be some independancy within the organisation - the person who makes the airworthiness review recommendation should not have active involvement in the continued airworthiness management of that aircraft. However this only applied to aircraft being 'managed' within a controlled environment. The ARC signatory can be same person as the certifying engineer for whatever work has happened at the ARC renewal.
Vee-tail
You are techincally correct that the review period is only supposed to be looking at the time since the last ARC renewal, however a CAMO has to be certain of the facts, and most are not prepared to risk overlooking someone else's mistake. Albeit for a simple aircraft establishing the status of the aircraft does not take long, and as I previously stated there is nothing massively new about part-m