1. How far do I pull the seat away from the bight approximately, and do I need a rolled-up towel/pool noodle to hold it there?
No, you shouldn't need a noodle or towel.
Pull the base (the square part that sits on the seat) out a bit so it is not smashed into the seat bite. Another trick: Get behind the seat (move the driver seat up as far as it can go/tilt out of the way) and use your hips to push/put my weight into the seat so the edge of the SEAT (rather than the base) is pushed into the back of the vehicle seat.
I can't tell from the photo, but it looks like your seats recline. If the seat reclines, lean it back a notch or two, buckle in the CR and then move the seat back to upright.
Just curious, is there a reason you are not installing the seat in the center position?
2. What's the best way to judge the 30 degree recline? Is there any point of reference I can use?
I would start at 45 degrees - assuming you're using te RF tether to get to 45 degrees, since that is easiest to check: fold a piece of paper to make a triangle, put the long edge of the triangle against the seat back of the MA so the point is on the seam at the bottom. The top edge of the triangle/paper should look flat or level to the horizon. From there, loosen the tether to allow the seat to move back up to a more upright position.
3. Does the harness height look okay? It seems to look different based on what angle I look at it from. The tech had said rf it needed to be in the bottom slots only, and ff in the top slots only, but I can't imagine why Britax would bother with all those other slots if you couldn't use them, right?
The tech has not read the manual for an MA (or any other seat for awhile). ALL slots in an MA are reinforced, all slots can be used either FF or RF. From the photo, it looks like the harnes is exactly at her shoulders so it should be fine (RF harness should be at or below shoulders). The slot below looks like it would far below/behind her shoulders - it's hard to tell from a photo as the angle can be deceiving.
4. My car is a 2000 Mercury Mountaineer, and my seatbelts lock when I pull the shoulder belt all the way out, then ratchet back in. That was actually giving me a lot of trouble.
Solution: pull the seat belt out slow/gentle, buckle & tighten seat and THEN pull the seatbelt all the way out to switch the retractor and tighten again.
You mention you have some difficulties with your hands, it would probably be helpful to have a 2nd person help you. The other person can hold the shoulder harness of the belt to prevent it from switching, help thread the seat belt through (one person on each side of the seat, as well as help pull slack from seat belt to fully tighten it while you put your weight into the seat. Even with two fully functional hands, it is easier to install a seat with 2 people.