oh good...another obsessive car gutting!
Maybe I should show my wife your post so she knows I am not alone...
You will first need to remove the trunk lid, if you havent done so already. Remove the rear seat interior panels, and trunk panels too, if you havent done so already.
Then just look around, there are a bunch of nuts that hold the trim down from the inside. Mine had some rectangular shaped 'washer's too under some of the nuts, so look around...
Be careful, this trim is plastic. I would suggest removing the trunk hinges first since these are 'over' the trim.
If you are painting the car you are correct removing the trim is the only way to avoid a minor tape line. The only way to remove the trim is with the top off so you are correct to do this 100% you need to paint the car AND replace the top immediately after the car was painted.
If you are going through this much trouble I would also suggest you paintless dent removal the car before bodywork to avoid filler, and also be sure to get a no-orange peel like glass job.
If you happen to need a non-sagging rear LX bumper I have a NOS one, new in the box.