1. Earlier in this thread, didn't you say that the rattling sound was accompanied by a loss in power? Is that not the case anymore?
2. Did you change the transmission fluid before losing 4-5th synchro? If so, what did you put in there?
3. Did you check the freeplay in the flywheel yet? Only two bolts hold that inspection cover on. It only takes a couple minutes. Also you can take a picture of what's inside and the boys here can likely help with a visual, if you see something worth sharing.
4. The flywheel is always moving if the engine is running. So clutch in or out doesn't engage or disengage the flywheel, but rather the clutch disc which turns the input shaft to the transmission.
5. The cold start advance adds 3 degrees to the base timing, which does help cold idle. One way to tell if your timing is retarded is to drive away while it's still cold-- sometimes it runs better cold than once the cold idle/advance drops down. If that's the case, you're retarded!
Edit: of all the words this site is censoring lately, I'm quite surprised I got away with "retarded".