Thanks so much for the helpful insights, Castlemaid, and for the lovely compliment too. Rachel's come a long way and I'm very proud of her. We're far from where I want her/us to be, but we're getting there.
Unfortunately, keeping Rachel moving to avoid dog/stranger encounters on our walks didn't work well. Neither did treats, a ball, or inviting her to play. In fact, it often seemed to ramp up her reactivity rather than dissipate it. (She often reminded me of a couple of hot TBs that I used to ride as a kid --- movement often ramped them up, at first, rather than relaxing them. Had to change my thinking/approach with them too). So, I thought, "I need a better way to reach her so we can have a different conversation about things..." which led to my decision to buy and use a prong. (A first for me) I used it less as a correction, per se, and more as a reminder ("Hey Chick, pay attention!"). The first (very mild) correction got her attention immediately whereupon I stuffed her face with treats the nanosecond that she looked at me. LOL. It may not be the proper
use of a prong, but it's working for us. Now, just the fact of wearing a prong keeps her attention where I want it. That combined with a pocketful of treats, liberally dispensed at the hint of a try, and we're doing better.
As to this new, odd behavior, I'm now thinking that it is anxiety. She doesn't engage in resource guarding. Well, not any more. Early on, she made a couple of half-hearted attempts with my older dog, Gunner (e.g., cutting between us when I was petting or grooming him). I shut that down pretty quickly: told her 'No,' put her in a crate, and continued to groom/pet him in front of her. Everybody's happier/better about things now.
More to the point, CSchmidt and Castlemaid, your comments reminded me of a couple of incidents that I'd forgotten. For example, on her first visit to the vet, Rachel was clearly anxious (panting, some whining, looking about) even though it was a very quiet area. As we were waiting to be seen, she suddenly draped herself on my lap. I patted her, had her sit (leaning on my leg) and kept my hand on her while we waited. We've stopped by several times since then to get her weight and just to hang out in the waiting room (the vet welcomes people doing this which is one of the reasons that I use the practice). Each time, she's less concerned when we do drop by. We go directly to McD's for some chicken nuggets afterwards which helps, I think....
Anyway, apologies for the length of this. But, that's what happens when somebody says something interesting and gets me thinking. So, back to the drawing board and walkies...
Many thanks again!