I certainly haven't had that amount of time to play yet alone platinum, I don't think I will either, I don't usually do it I just take it easy like previous game DA games. Couple of questions though, you say you can do it in one playthrough..  the kill certain amount of demons, do you know if it's just one or over all characters? I think DA:O had a Darkspawn trophy like that but it seems to be your character only and not over multiple characters, just that one so never got them.

On a side not, 55 hours and I just finished the story (last night), final boss was really easy.

This game is a mix.

Some need to be in one playthrough some are combined.

For instance you have to recruit 10 agents in one play through, kill 10 dragons, harvests Rebs from personal garden and a few more,

but the kill 1000 Demons and 2500 enemies trophies are cumulative. I got both around lvl 7 on my nightmare play through.

The trophy descriptions are pretty clear in this game about what is in a play through and what is cumulative.

I platinumed Origins and DAII. I haven't decided whether I will platinum Inquisition yet. It may depend on how much grinding and tedium is involved in some trophies.

No grinding at all per se. just clearing zones, the difference is on normal you can do the story with clearing less wilderness zones, to beat the level 23 dragon on NM you need to pretty fully clear all zones, 

when you beat the game, do you get a new game+?

Ya I did my first run on hard, and now I'm pretty close to the end on nightmare. Would have been done by now but I had to go visit family for a while and started playing Sunset as well.

Btw for all the DA vets reading this who haven't played Inquisition yet, I recommend playing Nightmare with friendly fire on, and don't roll Knightenchanter or Tempest. Otherwise it's way too easy

No. You just keep playing, you should beat the game on normal at 18 or 19 but should be lvl 24 or 25 once you do all there is to do. I recommend waiting til lvl 20 if on hard or NM.

Cool, I'm more of a Story guy anyway but it's nice to know these things. I love the world of DA and it keeps getting better. After having finished the story, I am hoping they bring another out soon or at least decently priced DLC.

Although, on a side note, the Dragon one says 'Slay 10 Dragons in single-player mode.' doesn't mention 'in a single playthrough' like the others...

I got the Slay 10 dragons achievement on my second run. The only dragon I haven't killed on my second run is one that I killed on my first run.

I got the dragon trophy at 8/10,  assumed the 2 you fight in the storyline counted towards the trophy but not the collect quest. I killed 5 of them on my normal playthrough so that had nothing to do with it.