I honestly don't know the answer to any of questions. By dogleg rail do you mean the Beryl mount?
I'm not sure other bayonets would work on the Tantal due to the shape and length of the muzzle brake.
1) The tantal is 14x1 LH threads, but the threads are set further back so normal brake don't work. You will have to get one custom made or suck it up with factory one.
2) I just sold my dogleg, and currently use an Ultimak on my Tantal. I find it to just feel more stable because of its design. The Ultimak also allows co-witness with appropriate low red dot sights (bushnell trs-25, aimpoint h-1, t-1, r-1). Just keep in mind that the Ultimak requires cutting the top portion of the handguard retainer in a permanent modification.
3) No idea.