Re: 6, there's precisely one area that you can endlessly scum randomized zones. Downside to it is that it will eventually kill you -- the guardians of the place have something particular to them that can occasionally make them deadlier than the final bosses

There's also ambushes on the world map, both in the early and late game. Downside to those is that they'll probably eventually kill you

T4 does have some scumming options, but they tend to be incredibly high risk. I don't recommend them if you're trying to not die, heh.
That said, there's six tier one dungeons accessible by everyone, four tier two... you should be able to get pretty comfortable as far as experience goes if you feel like clearing everything out. You might check here, though it says it's incomplete. Seems pretty solid for the west, though, so it might be something good to get you started off.
Re: 2, you might consider using the SVN version. It'd let you download once and then update individual files as DG and company update th'files themselves. Downsides being you wouldn't be able to use the character vault and, well, you'd have to be willing and/or able to figure out how to compile T4 on whatever rig you're using. But, it'd definitely save some bandwidth over the long run, and help cut down individual download periods into more bearable chunks. Plus you'd get access to cutting edge changes

Re: 4, I think the Furyband site has a mirror for 2.3.4, which looks like the last T2 version. Lemme see if it works real quick... yeah, looks like it. Try here. There's probably a few other places with mirrors, laying around somewhere. Checking, The angband ladder does have a 2.3.5, so you might prefer there. Also give a gander to the T2 part of this forum. It's down a bit from the T4 and T-Engine stuff. Looks like there's a thread for finding the latest stuff down there, so you might have some luck with it. E: This post seems to have a link to a working build of 2.3.8. Just downloaded it and fired it up to see if it'd run and it didn't seem to spout out any errors or nothin', and I hit Bree without any problem, so that might be what you're looking for. Unless you want to try compiling, as there seems like there's a 2.3.10 for linux or somethin'.