Mine is about 5yo. Seat back pull cable clutch has broken.
Is there a better seat for this? More padding for a boney butt. Or should I just fix the issue and if so, what is the part called and where can I find it?
Is there an aftermarket fitable top drain plug?

I’d get another Phase 3 seat , one bony butt to another. Or Surf to Summit makes some nice ones.

Sea-Lect makes a drain plug kit for kayaks. http://sealectdesigns.com/groups/1831-drain-plug-kit

The boney butt issue could be handled separately from the seat. You could add some padding, like the Jackson Sweet Cheeks (http://jacksonkayak.com/jk-kayaks/whitewater-kayak/zen/accessories/)