1) When do you restock potions? At the end of your turn or the end of the level/floor?
You start with one white potion cube. You get another every time you claim a loot item with a potion. You get another (just one!) every time you level up. You may use any white cube for any of the potions that you have discovered (at least healing).
2) Aside from what the floor tells you to do when it comes to spending time /card before you enter the floor, do you spend two-time THEN create the four-door setup AND play a door? OR do you spend two-time, create the four-door setup THEN spend two-time again THEN open one of the doors?
You spend 2 time to Explore, which will begin the game by giving you 4 closed doors. You then spend 2 more time to open one of those doors.
3) When do you replace the doors you've already resolved. After you spend another two-time OR right away?
You do not automatically replace doors. If you want (or need) more doors, you spend 2 time to Explore. That will give you new face-down doors up to a total of 4 doors. (Thus, if you spend 2 time to Explore when you still have 2 old doors, you get only 2 new doors.)