Mid door trim-Actually the best thing is to remove the trim once a year and do an inspection. Any signs of rust or chipped paing in the holes or inside the door panel should be cleaned with a dremel and wire brush and repainted with top coat enamel (basic Volvo OEM Touchup works fine). then let dry and wash/wax and reinstall the trim.

Buy a set of clips at the dealer ($1 each) and replace the ones that break.

Any goop behind the trim will only attract crap and aid the corrosion process.

Same goes with Tyvek or any liner. 1) Just try getting it cut clean and installed w/out any showing, 2) It will not work unless it is glued to th trim and then you have to peel and clean before reinstall PITA, 3) The looseness of the trim actually allows water/dirt/grit to flow under the trim abd away from being trapped.

Lower door trim- This is held in place by several prongs off the back of the trim and uses "some" 3M tyupe tape (1 strip per 1/3rd of the trim height). Clean door face well with alcohol and apply tape keeping outside backing strip in place, clean inside of trim and scrape off old tape. Carefully line up the prongs then peel cover tape and install trim. You should also do inspection, cleaning and touchup as well before reinstallation.

BTW If the trim (upper and lower) is faded SEM Flat Trim Black will do wonders and restore the OEM look. When waxing use Saran Wrap over lower trim to protect it and same for mid door trim. Use Mothers Bumper dressing on Bumper valance and lower trim with wax paper slid behind trim to keep Dressing off door panels.
1 application should be good for 4-6 months in Avgerage Acid Rain environ.