You can clean the bearings by just separating the outer from the inner as they are tapper bearings. has yours got the removable caps over the bearings?

Yes the thread is there to preload the bearings, how much thread is left? Not sure how easy it will be to rethread as I think it will be hardened. Is there not enough thread to cover the lock nut?

I did mine a while ago but don't think there are any collars. there is the main shoulder at the nose end but then the bearing inner races are compressed to preload the bearings.

I have to go to the workshop tomorrow to check a few bits for the mill so I will have a look at the spare spindle to see what its like.