#1 you probably can not get a grid screen - If you can it will not be a Nikon one.
D70 grids are electronic not a etched on the screen so unless the D50 has the connections. No
Beattie makes after market screens for cameras but if they do make a D50 screen. I think it will be cheaper to sell the D50 and get a D70. There may be a bunch of them used when the D200 is available. The Beattie screens used to be about a $50 plus installation.
Call Nikon at 1-800 645 6689 for a better answer.
#2 yes but then you no longer have iTTL flash -- Set the flash to manual and you will have to work with guide numbers for exposure - The preflashes are only a problem with some people, most do not blink and it may be that they see your finger get ready to push the button. I have this happen on cameras that do not have a preflash also.
#3 buy a SB 800 and use it A mode (or any of the older SB's), AA or M mode.(it will calculate distance for you.