Here is the information I am aware of and will try to answer your questions.
1. Did only the friend or both of them make it back to the bar?
Officially, from the "Friend" and family the last time she was seen was at the Shell station. The "Friend" made it back to the bar and located the phone, but Danniella never showed up and he went home. It wasn't till late that the phone was handed over to the family by the "Friend". Then it was turned over to the police.
2. Where are the other two points of interest?
I think the first one is going to be the original bar, even if she got lost, Mobile is fairly easy to get around, so I can't see her going home. The only other place she may have gone was to Dauphin Street to some of the bars there. Maybe she didn't want to go all the way back to Dublins and wanted to continue on Downtown?
3. Which way did they go out of the Shell?
This has been a sticking point for me since the beginning .. That information has never been released.
The typical route would be West down 90, then North on Demetropolis Road, which turns into University, the West on Old Shell to Dublins.
4. Was the GPS was intentionally disabled or accidentally disabled?
I can't see GPS being disabled accidently. Typically the fuse box is hidden under the dash and has a plastic snap on cover. It would take a bit of work to knock the cover off and knock the fuse out. Typically you have to use a fuse puller to get the fuse out, because they are so tight.
Here is a good website that shows how to disable OnStar and GPS. (Same fuse)
This could easily be researched, and completed in less than 5 minutes.
Wolf's Project Files - Completely Disable OnStar On Your Chevy Cruze