>is taking the trim off and filling the cavity with sealer a bad idea?<

A very bad idea. It entombs water in the cavity, never able to dry out. Notice how open the w/s pinchweld is? This allows for unimpeded excavation of water; likewise, for the rear glass.

By the way, forgive me if my image size is too large because I’m computer illiterate, and I don’t know how to make smaller file sizes.

I have been drying the rear pinchweld this way for decades.

Here’s the theory behind using the clip removal tool. Shop tip: You can release from either side of the clip, the red clips, but you don’t need to flex the clip as much if you snatch it on the tab side.

(The white clip, a different style clip, is in the mix because of a factory defect. See if you can figure it out.)

You may run into a couple of problems. First, I think the moldings are glued on. I could make short work of glued-on moldings without damaging anything, but the wear pattern on the clip tool shows that this wasn’t my first dog & pony show. I would say to take your time, but taking your time and technique are different arenas. Technique comes from taking your time, and phucking up lots of chit as you develop ‘technique.’

Second, the changes are better than good that you have a bigger problem than a simple seal job. Most likely, the rear glass needs to come out and do some metal fabrication. Also, packing a bunch of sealer in it will be pissing in the wind.