I have one in 454. People who are accustomed to shooting big bore handguns don't hand it back after one shot. It is surprisingly easy to shoot well given the sight radius. 300+gr bullets can be easily pushed over 1000-1200fps even in 45 Colt brass and I see no need for more out of a snub. More...
There are many, many good holster makers that can make pretty much anything you want. I would start with simplyrugged.com. He shows a half dozen models for the 5.5" Redhawk, probably half of them can be used cross draw.
Don't know if they are the "best" choice but what are the other choices? Standard Ruger rings can be used. I've done it and so have others. I have a MKII RSI in 308 set up with a peep and a 4x fixed scope that I interchange with no loss of zero. The advantage of the Warne's is they have a lever...
I guess it depends if you want that short a barrel or not... The only "hassle" if you reload would be that brass isn't as plentiful and likely more expensive. You definitely will have significantly more velocity with the RCM over a 308 with any given barrel length. Assuming the example given is...
Your best bet would be to contact Hornady. They developed the ammo using their (Hodgdon) Superformance powders although other powders will also work. Perhaps a better question would be what bullet weight & velocity do you need for your intended use. Hornady claims 75 to 100 fps difference...
...and that was based primarily on the case capacity and powder they used to load it. The only way to know what the actual velocity is in any barrel is to shoot it and measure it.
Original MKII floorplates were not steel. I believe aftermarket steel was available and the engraved ones through Ruger were steel. That changeover was part of the Hawkeye configuration.