So I was there and it was a great show. Dave was completely normal.. lol. This is a very strange post. Overall, great festival set. The band even went over the allotted 2 hours and played past curfew. The band admittedly was a little rusty, I think because this was a one-off show and not mid-tour, in that they missed a few cues here and there. But it was actually pretty cool to see them rebound and communicate with each other, and laugh it off afterwards. Dave was having a great time - he specifically mentioned how they were happy to be there and get to hang with friends backstage from other bands. Maybe OP is mistaking Dave reacting to the cold a few times in a joking manner, I'm not sure.

"Dave appeared pretty “messed” up and could barely speak to open the show." No idea what this means -- when they came out onstage, it took them a couple minutes to finish getting things set up before they started playing.. totally normal for a festival. Dave a couple times was ready at the mic and backed away while Stefan still had to get some things figured out with his rig. Completely normal.

"During this time, he constantly had his back to the audience, walking on the side/back of the stage (out of video feed view) and there were extended and more musical riffs within the first 45 min than normal." --- He just walked to the side occasionally when bandmates were soloing as he normally does to let the attention be focused on them. As for "extended musical riffs," I have no idea what OP is talking about. You mean solos? This is the Dave Matthews Band. The set was actually way less solo-y then normal as they were sticking to mainstream songs for the festival set.