You might want to consider talking with Craig at Avalanche, when I got the Avalanche fork cartridge for my Lyrik he said he could tune the Super Deluxe for the Cascade Link, my weight & riding style as well. He isn't afraid to dive deep or tell you his opinion, so be prepared for that.
Might be easier to make a little rubber boot for the seatpost/seatube interface. Or even a little caulking would probably suffice. Guessing that's where all the water is getting in.
I sometime had to put some force on them to break them free. Try with a longer wrench if possible, just be careful to have it completely seated toavoid stripping the head.
Those sometimes corrode a little. You could try spraying something from the other side.
Easy out, hardware store like Ace should have replacements in the right length. Had one seize on me when I changed links.
I did find a nice cheap easy out set for $20 ish.. it's come in handy more than a few times already. It's officially called a "damaged screw extractor set"
-One Up carbon bars -Cascade link -Marzocchi Z1 bomber fork set at 170mm -One Up chain guide/Bash Guard -Race face Turbine 35mm stem -One Up EDC stem tool
I was wondering if anyone has any experience using the Cascade link with a coil? I might be able to swap with a friend of mine who wants to try an air shock on his bike and our shock sizes are the same