My collar showed the remains of some fairing compound on the inside , down to the teak trim. My trim had been removed and reattached sometime in the past, so it was no longer faired to the collar. The rest was flaking off, and was quite thin, so I basically ignored it.
I left the collar in place. It made it real easy to seal the mastless deck with a piece of leftover "cocooning" plastic and a bungee cord. Didn't get a drop of water in all winter and spring.
The outside of the collar was covered with a lot of white aluminum corrosion. I considered priming and painting the outside, but in the end just cleaned it up with a scotchbrite drill wheel designed for stripping paint. Works much better than a wire wheel, and no steel wire bits left lying around to rust on the deck!
I too found that the upper shrouds had forks with 7/16" holes & pins, but the tangs on the mast had 1/2" holes! This is generally regarded as a bad thing to do, an it increases point-loading on the tang holes and causes them to wear unevenly and elongate. It seems that it is very difficult to get fork ends with 1/2" pins for 9/32" cable. And they're expensive. What my rigger did was to replace them with toggle-ends with 1/2" pins. These fit properly, and are less expensive.
Also, in a departure from my previous plans, I decided to try a premade universal mast boot instead of the tape, since I could slip it on without cutting before the mast was re-stepped. I figured if it didnt work, I could always cut it off and revert to rescue tape, but I couldn't go the other way around!