1) 12h30, but a lot more counting the reloads and stuff. Steam says 19hours, but a bit of it afking and I rerolled at lvl 3 or so, so likely 16hours.
2) I only needed help for the very last puzzle, and I wouldn't have needed help if it was more obvious you had to click in specific parts of the screen for the items to go in the cube. Like I had all the items but they "wouldn't fit", because I was clicking the wrong place

I also looked up spells early on since I dumped a lot of points in fire way before I got the scrolls.
3) 53/71secret, 3/7treasure, but I only really started full detective mode at lvl 7+, before that I kinda just checked for wall stuff if there was a visual indicator there should be one(a door with loot behind for example). According to the items missed list, I didn't miss anything from 7 to 13, but I missed a lot before.
4) My main char was 14.5 due to necklace of xp, others were 13 but fairly close to 14(like maybe 10mins of grinding in the last room). I kinda wish there was more xp in the game as I feel most of my characters weren't "complete". Only my archer felt kinda done, mostly because all you need is missile weapons and I reached 50 in that before the end.
5) 9.5/10. I wouldn't give a perfect note because the controls were sometimes clumsy(no way to keybind attacking, no way to move the frame closer to the middle so I don't watch a corner of my screen in combat), some balance issues between like spell magic schools or classes(protection vs evasion tanks for example), some puzzles/enigmas were annoying like the platemail on lvl 5(I think?) where you actually have to go lower then back up or some switches on walls that don't do anything apparently because what they activate is too far for sound clues(one button on the spiral corridor at the end of lvl 9 where there were a couple of ogres, couldn't figure what that did at all), a bit slow to give bow+arrows for archers and disparity between melee weapons, lack of stealth to support a backstab heavy build without relying on freeze or invis.
All fairly minor details and I still feel it's one of the greatest game I've played in a while.