Bonanza:
You've got a few answers already, but I'll add mine anyway.

1. Yes ...but I prefer the 2 3/4 case as it is far less likely to ever be accidentally loaded with a full nitro cartridge, and it requires less wads.
2. Yes I use a Dremel with an abrasive wheel, easy.
3. Yes ...but full-length sizing dies would probably need the bigger press with the insert. With this low-pressure straight-wall cartridge though, you will only ever need to neck-size during normal shooting.
4. 90 to 95 grains Varget behind a 650gr jacketed bullet, with a filler-wad (either a big tuft of dacron/kapok, polyurethane foam, or saddle-felt to completely fill the case) should give 1650 to 1700 fps at 8 to 10 tons. Start at 90gr and work up using a chronograph to get the required velocity.
5. Can't help from this side of the big pond.

I can't find that Sauer shottie on the Thad Scott website, has it been taken down? If it is a solidly-built gun with a cross-bolt and heavy nitro-proof then it would probably be fine.

I'll take some close-up photos of mine for comparison when you blokes get a bit further down the track. Looks like there will be enough of us for a "12-bore to .577" club here on NE before too long!