1) No, the Rem cartridge is much shorter and belted whereas the Rigby is magnum length and unbelted. Same ballistics.

2) One is for rifles and doubles with a belted (H&H) and the other is exclusively for doubles (Flanged) and they're not interchangeable.

3) Yes

4) No, they're not in production any longer.

5) It depends on the individual user. For double rifles, the ejector is quicker for a reload but a little more difficult to open some actions.

6) Again, for double rifle users, it depends on the individual as to preference.