1. I guess you want to have pictures every few seconds. You can just divide the overall estimated time to tidy up by the number of shots you need for the final video length and that will be a good basis for your time interval.
2. Handheld will be a big issue since a correct alignment will be extremely difficult up to impossible. I recommend to borrow 2 more cameras (e.g. GoPro's) and have 2 of them in fix locations and the third one on a tripod that changes the position to follow the action, e.g. placed some hours in one corner and then in the other, to get some “close-up” material. With these, you will be able to make your final video more interesting by changing the views.
3. The changes between sessions will as well have the same issues as explained in 2. With simpler cameras such as GoPros you could probably find a more durable solution, e.g. by nailing a wooden board to the wall and placing a cheap GoPro mount on this board. This could stay there between sessions.
4. Night may be a problem since the light of the bulb may not suffice for reasonable shutter times. You could anyway need higher ISO settings since the light through the windows may not be enough. On the other hand, I would try to utilize the changing light conditions to place perspective changes (e.g. change between cameras) between the sessions (but not only there).
5. Yes, that makes perfect sense. For the video you do not need the full resolution, therefore you will hardly see any negative effects from dual-iso. However, I would recommend to practice dual-iso beforehand, since a lot of experience is needed to operate it well.
6. I would skip this question and do several perspectives by borrowing some cameras. Even camera phones with timelapse app may help, and these could easily be placed accurately over several sessions, if you construct a proper mount, e.g. from some wooden boards.
7. Practice a lot beforehand since good time lapses require a lot of experience. Try “filming” a typical day in the office or your weekend activities etc. This will help to get something useful later.

I wonder if there are really several session required, it looks like it should be done in a couple of hours, if you do not have to do it alone.