I have removed everything from my existing powder room sink, with the exception of the PopUp and the flange from the sink-hole... The PopUp (body) just turns in my hands as does the flange, but I can't remove it... Do I need to dis-connect the drain from the wall entry or not?

There is a nut under the sink. You have to take 2 sets of channel locks. Put one on the nut and the other one on the drain itself. Turn either the nut or the drain to losen the nut. Yes, I agree, it is kind of difficult to do since you have limited space.

Or, take hacksaw and cut it between the nut and the sink.

You can also take hacksaw blade and cut that flange from above in two places. Then, insert skinny flat screwdriver under it and gently bend it in upward direction. It will get come out.

Yes, disconnect the trap and the lateral drain into the wall. This will leave you with the vertical drain and the arms that actuate the pop up, remove the arm by unscrewing the cap on the end of the arm as it goes into the tailpiece. Maybe if you buy your new sink and drains you will better see how they go together. Read the installation instructions and then do them in reverse.