I have learned a great deal since then.
First, if you have a spa or pool, the forum to check out is Trouble Free Pool. Those folks know their stuff, and unlike most pool/spa forums on the web, there is plenty of traffic, and just about all questions will be responded to promptly.
While draining the spa this time (only the second full drain down), I discovered that the conduit fitting in the light niche had been corroding. When the water level got down to the niche and it started draining, thick rusty water started coming out. I pulled the light, and a LOT of thick, opaque orange water came out.
So that changed things a bit.
First, it explained the iron contamination of my water. The clarifier and metal sequesterant I was pouring in were just barely staying ahead of it.
Second, I now believe that it was responsible for the fast depletion of bromine residual, and with the bromine being eaten up, other stuff was going wrong, too.
So I decided to postpone experimenting with other sanitizers, and give the 2-part bromine another chance. All bets are really off if you have something rusting away in your spa.
I repaired some more grout, just using the latex grout-in-a-tube that seemed to work fine last time.
Found a nylon conduit fitting to replace the steel one.
Nancy and I went over the rust stains on the inside with ascorbic acid and oxalic acid.
Upon refill, it was instantly apparent how bad the water had gotten. Since it had developed a tint rather than cloudiness, and it did it over a long period of time, we had gotten used to it.
I put in one packet of HTH Brom-Start, and a couple of ounces of MPS to get it started. Couple of cups of baking soda for TA, cup and a half of borax, and an ounce of dry acid to bring the pH back down.
I had installed a skimmer sock to keep podicarpus leaves from getting to the pump basket, and it turned orange from the rust that was still stuck to the walls, so I added one dose of sequesterant.
The water looks gorgeous. Very little bromine odor, too. The dry skin seems to have been related to lower pH, so I keep the pH up. Wanted it on the lower end for a while to free up any remaining rust, but it usually sits around 7.6.