1) Not sure.

2) Pretty sure yes. The next skill in the path that makes you more proficient in scrolls is quite worthwhile if you at all want to depend on them (which you will). And the edge that prevents wasting scrolls is pretty useful for thieves too. There are some really useful scrolls around (aura, stoneskin, defean, haste, etc.), and it can be hard to get them so you have to make them count.

3) Drow/elf is going to make you frail, but biggest issue may be the xp penalty. Grouping as a thief isn't so easy-- expect a long journey as an elf or drow, with marginal pk range and to be easily killed if caught in the open.

4) I don't know much on infiltrate, but I know some very valuable/interesting doors and chests can only be opened with advaned lockpick. Found it very worthwhile unless you are designing a village pker or outlander that won't want much gold. A thief with lots of gold = near infinite healing potions and other preps, among other things (including lots of friends). Great for exploring too, combined with peek and gentle walk, you can go a lot of places most can't.

5) My guess is there's better ways to spend the points. A lot of the basic optional skills are really useful. Envenom will rarely be that great, especially since you'll want to have a selection of the cool weapons (which you can steal anyway) like Wave Dancer, Weaver of Doom, Deluge, Wasp Blade,ghostly dagger, etc. etc. Finding weapons to attack vulns won't be an issue. Something like cheap shot or even legsweep can get you a lot of kills. And the skill that lets you seperate (or is it isolate?) enemies away from their group is really useful against the Tribunal since you can get them away from their guards before you backstab them. Lots of fun options.

6) Kind of gets at whether you want to be more defensive/explore build or offensive/pk build. Third attack makes a pretty big difference in a lot of situations but like I said I've never studied lore of the ages. If you're going primarily thug, I think up through third attack, scrolls, advanced lock picking, and whatever other utility skills you prefer for your cabal/build will be a lot fun.