Love the video. I didn't really come to any conclusions as I had a ton of things going through my head as to what it could be. I will say this though....Miko was obedience trained and he HATED to walk on my right side. Vogue was a show dog and dislikes walking on my left side. When I got Koji, because of the weird issue with Miko, I made sure to walk him on both sides. Granted, it helped, but because Miko prefered the left, when I'd walk both, Koji would be on the right. Guess which side Koji prefers? All three dogs won't potty either, unless they are on the correct side and the grass is the side that I'm not on (assuming I'm just walking). So it seems that they do have a preference by whatever we taught them.
Therefore, my conclusion is I think it has to do with you training obedience first, especially if you didn't make sure to mix it up from the time Cleo was a puppy. I would start walking on her on the opposite side all the time. Get her comfortable there. I'd also go back to basics. I noticed in the video, you tend to hesitate on the side she's slower on and even you're not walking very confidently. Wonder if she's picking up on that at all. Your arm/hand placement and bag full of treats don't seem to make any difference. Start over, and if she pops, go back a step. Meaning, take it slow. Also, while you're doing that side, you can start working on proofing things on the "good" side. Like, being farther away from her (maybe she needs more room on her "bad" side?), going ahead, going behind, going away at an angle, etc. Do this slowly, but at least that way, she doesn't feel like you're pushing just one side.
BTW, for the fun of it, I tried to do agility with Miko when I was doing it with Koji (after class, bars really low cuz he was older by then). Nope, still absolutely refused to go on my right side. We did Rally-O instead, perfect! Haha.
Good luck. Sounds like this is a common problem that you just have to work through.