: 1) Do I need to sand before the final coats? If so what grit (320, 600, 800)?

It is too bad you waited for the final coat. You could have skipped the sanding. But, YES, you are now stuck with sanding. The sanding roughs up the surface to create a mechanical bond between the coats. I would think that 120 is probably enough. The factory might give you some guidance because too fine a grit doesn't leave a surface that is rough enough.

: 2) How many coats of cross-linked LPU should I use? I was thinking two would be good.

I would go for two coats to make up for the material sanded off.

: 3) After the cross-linked coats, what do you do about drips?

What drips?? If you have any, take care of them now. Thinned material results in more risk of drips. why are you thinning?

: 4) Last question!!! Do you thin the cross-linked coats, or is gloss higher
: with out the addition of water?

Sorry, i can't help you here. I am very happy with three coats of unthinned. It is plenty shiny for me.