Welcome tot he Forums!
I would caution you on dismissing the windows as a contributor. If the window seals are old or cracked, then the windows might be leaking and helping with the problems. You can also be getting leaks from up above (ie, any of the roof penetrations, especially the plumbing vents), that allow water to run down the inside of the wall and pool in the C-channel. The "stack" windows up above are also notoriously leaky. Since you have the walls opened up, I would advise you to set up a sprinkler on the outside to really soak down the outside of the trailer and watch closely to see what is actually leaking.
Now, if you find that the Olympic rivets are leaking, then maybe you can stop it by gooping some more sealant on the rivets from inside. The "best" fix might be to drill out all the Olympics, separate the sections of skin, clean up the interface, then put sealant between the panels before reinstalling, and dip each solid rivet in vulkem before bucking them in place. You might still end up with a leak after all of this, though.
The gap in the C-channel is normal. When you talk about a "hole" at the top of the trim, I am not sure what you are seeing--is there actually a hole/gash penetrating the shell, or just a separation between the trim and the shell that is funneling water toward the rivet line under the trim? The picture just looks like a typical poorly fitting belt trim. It doesn't need to be (certainly isn't) water tight. That belt trim, and the banana wraps, are held on by simple, leaky, pop rivets. These might be even more leaky than your olympic riveted panel repair.