As TightSlot says a lot of this will depend on your airline's SOPs.
From my cabin crew days...
1) I assume you're referring to reduced crew compliment here? Then yes you can, provided the legal crew
ax ratio is maintained.
2) Certainly was my airline's policy.
3) Yes there is no reason why not. Tablets and laptops must be stowed securely for take-off and landing though, be it under the seat, in the seat back pocket or in the overhead lockers. This was a grey area, but many people believed them being left on laps was not a good idea as they could easily become projectiles during a rejected take-off. I would imagine there is an official line on it now.