Ok, so I completed the questline on the DG side.

1) If I let Serena turn me into a VL, can I still convince her to cure herself? (I did all the proper dialogue options before, I just wanna know if her turning me and her curing herself can both be done)

2) I have a pretty invested character (leveled in the 80's, tons of hours/progress), but think he would be more suitable as a VL on the Vampire side. Can I just delete the DG add-on and download it again, and basically just choose the Vampire option? I'm on 360, btw. Will my save be fine? Will my items from DG stay there as long as I both delete AND redownload without loading the save?

1. I dont see why not. Unless they have taken that into account and put in some sort of check for that, but if you go through the dialogue options and its still there to have her cure herself, then yes it is possible.

2. Possible... technically, yes. But no, you shouldn't try it if you are invested in your character.