I received the report on the x-ray I had of my hip and - good news - there is nothing wrong with the joint or bone. Which still leaves the question as to why my hip is having difficulty with full range of motion in one direction. I've been trying to pay attention to how I sit and stand, bend and lift, etc. to see if I do anything differently on that side. I'm diligently doing my initial home physical therapy exercises, making sure I sit with my knees lower than my hips and picking walking routes that minimize hills per instructions.
The truly good news is that whatever is wrong doesn't limit day to day activities including my ability to take a good walk in the neighborhood on this sunny day. It's just that if I sit and put that ankle on my other knee then the knee on the bad side really won't go down to the side. If I wasn't exercising regularly and doing that stretch I probably wouldn't even have noticed it yet but it does hurt to move it that way.
I did get the PT referral and it will be interesting to hear what hypothesis 2 is now that there seems to be no arthritis or other joint problems. Have to wait a couple of weeks to start PT but I'll keep doing my 3 little home exercises, sitting properly, walking on routes that minimize hills, and going to Pilates and hoping it gets a little better in the meantime.