I really like the Lyman's .50 cal Deerstalker. I think I'd prefer the stainless model. Looking over their info it shows the blued model has .520" grooves and shows that's it's ideal for RB, whereas the stainless has .510" grooves and is ideal with conicals/sabots. Both have a 1:48" twist.

Would the shallow grooves in the stainless model not work well with RB?

I would prefer to hunt medium game with conicals, but would like to try out RB, especially for small game. As I have a ROA could I double patch a .457" RB theoretically?

Should work just fine with a tight patch and ball combo. Personally I would be inclined to try a .495 ball.

I don't have the Deerstalker but I have a .50 & .54 caliber Lyman Trade Rifle. They shoot patched ball excellent and sabots. Also they shoot powerbelts, but I have not tried them on a lot of conical bullets out there.

Theoretically would the 1:48" twist work better with the shorter lighter conicals such as Hornady's 240 grn PA conical, Lee's 250 grn REAL, and 240-255 grn pistol bullets? It seems that if a 1:60-70 is ideal for RB and the very fast twists are intended for big heavy conicals the 48" ought to be about right for short bullets.

If pushing pistol bullets to 1600-1800 fps would 45 Colt bullets hold up or should those intended for 454 Casull or 460 S&W be used?

How would these projectiles work in a .502 x .520" rifled barrel?

I've found several different projectiles that shot well from a 1-48" twist PA conicals, round ball, pistol bullets and heavier conicals like the great Plains or FPB's.

Deep or shallow groove?

Well... not sure! I'm shooting a Traditions Woodsman Hawken. I'm thinking deep groove.
Cayugad has the same rifle in a Flintlock, I believe. Perhaps he would know?

I'd prefer a .54 with the slower twist and deeper grooves...Then you don't have to worry about what projectile, just go with a round ball and you are good to go...

I've been thinking about a .54 with PRB. But the deer here are fairly small, though we both want to move eventually. And it just seems that a .50 gives many more options.

The .50 does give more options, but if you want to shoot a PRB. The .54 is better. It's good for any deer, and elk if you keep it within it's limitations.