1) maybe. either deepen it gently, or let it deepen and just keep investigating.
2) when you 'switch' is up to you. in some people's practice there is no switch.. samatha just kind of blends into vipassana. or, as in mine for example, there is also no switch.. because i just start with vipassana. i think a lot of other people do too, highly recommended. best characteristic to start with = suffering arghhh. in suffering characteristic there is the world man, no one should neglect suffering characteristic in their vipassana practice.
3) personally i think yes, force it. start understanding how sensations (sense impressions) are happening one at a time, one moment at a time. start by understanding how the suffering you experience is normal and can be looked at without flinching.
4) in my opinion you should pay attention to what sensations make up the back that is tired, what sensations make up the desire to slouch, what sensations make up the slouching (if that is what you end up doing), AND ESPECIALLY what sensations make up that uneven annoying feeling

from your questions it sounds like you are definitely going the right way so keep going.