EMAW

1. You can estimate, but be as close and accurate as you can get.
2. In a crew environment, or as an instructor, you should only be logging the takeoffs/landings you performed. Yes it's a good learning tool to watch someone else, but your logbook proves your proficiency, currency, etc.
3. Depends on how you are defining PIC. I logged PIC as an instructor because ultimately I was signing for the aircraft and had final say on the particulars of the flight. However, depending on how the App asks, I usually separate it out. I.e., if they ask for PIC and DUAL given separately. As far as now goes, if I'm not designated PIC on the paperwork or release, I log SIC even though I'm fully qualified on the aircraft as PIC.

Ultimately it's up to you to be accurate and forth-coming with your experience. If there is a definite discrepancy, they'll find it and you'll have to explain yourself. Good luck.