I have a RR 9mm upper on my Bushy lower. I forget the model, but it's the carbine version with a flat top and short handguards. I also use the modified Sten mags with the correct adapter. I can't remember the name of the adapter, but you can find it on AR15.com. I think I bought it from Brownell's.
1. The hammer only needs to be filed a fraction of an inch. The part that needs the filing (polishing is more like it) is the top corner closest to where it contacts the firing pin. That corner is the part that will hang-up on the 9mm bolt because the 9mm bolt does not have the relief cut on the bottom. I actually ground my hammer down too far and had to replace it. On my second try I used my Dremmel to take off just a hair of metal. I didn't measure it, but the thickness could not have been more than 1/16 inch. Just enough so that the bolt clears the hammer on return to battery.
2. I use my stock Bushmaster buffer and spring. No mods whatsoever. Winchester USA (white box) 9mm runs great. UMC 9mm runs really, really slow. Must be lower pressure. (Semi-auto only. bummer)
3. I've only tried the adapter I have now. It takes a ton of milling on the adapter to get it to work with the Sten mags and to fit in the AR15 well. But, once you get it in, it works fine.
4. Maybe. The Sten mags themselves must be modified to work correctly. An AR15 style notch must be cut into them so that they will latch into place. There is no last-shot bolt hold open.
Modifying the mags is relatively easy. I did mine with the ever popular Dremmel and some cutting discs for tempered steel. The cuts are not beautiful, but they work.
The mag well adapter is held in place with three small set screws and the mag catch. It will stay secure for awhile, but prolonged firing will require you to tighten the screws. Some folks have a dedicated lower that they drilled and tapped to make the mag well adapter a permanent fixture.
I love my 9mm AR15 setup. But, if I had known about the Beretta Storm I would have bought one of those instead. My AR15 is great, but the work and expense to get it running was a serious pain. Less money than a Storm, but I prefer to pay and play. Call me lazy.
I don't really enjoy, or have any skill for, machining metal. The mags I did myself. The adapter I took to a gunsmith with a milling machine.
If you do decide to do it, read the AR15.com thread two or three times. Some of the original posts have been deleted so it might be hard to follow, but all the info is there.
I'll answer more questions if I can.
Good luck.
edited to add: My Sten mags use the standard AR15 mag catch. I drop the mags using the same mag release/catch that came on the rifle.