The 4 O-rings are not found ever I think. This was an attempt to fix the noise the trans makes in neutral but it didn't work and so they are never there. I'm not really sure of this info but I think this is the explanation I was given a couple years ago when I insisted there were supposed to be O-rings in the box.
I don't know if it is OK to use this trans as is. If it were me and I had to use it I guess I would. If you are correct that the spring was chewed up then it will be OK. But I can't say for sure that you are right. I want it to be right but I don't know.
The next time you take this apart will be faster. You know the drill and you own all the tools, correct?
And the next time you take it apart do a complete take down. Take all the shafts out. You do not have to replace any bearings that are still good. Make sure you check the large bearing on the front of the out put shaft. This one gives some trouble.
Other than the funky down shifting everything is good? Good luck with it.