I guess I'm confused: earlier you said this:
"Got the new collar/washer from autopts, had time today to assemble and see what I have. Slop/backlash is same - 1/2 turn each way (no surprise). Putting the washer in the pocket didn't change it much. Now that the spindle is riding against the new collar, up/down play is really evident, so slop forward/back, and up/down."
Exactly what do you mean by "up/down play"?
If its straight, and the threads are not sloppy in the main nut, I wouldn't worry about end movement up down at 10-12" away from the nut.
If the nut is worn, that could cause backlash, or if the screw threads are worn in a section heavily used that could also (Lathe and Mill lead screws commonly wear at center for instance)
Get the main nut out and thread in =t on the main screw. Make sure both are clean. try to see if there is linear movement as the nut is positioned on different parts of the screw. if any lateral movement is minimal and even, wear should not be an issue.
I'd hate to see you spend a lot on a main screw and discover that isn't the issue...