Thanks for all the feedback and commiseration guys. I was debriefing myself with a friend today, just to go over the situation and what I might have done wrong.
I was sailing on my usual lake, in between a starboard broad reach and a beam reach. The winds were moderate, I was sitting . So my back was to the wind. However, I did have other hints that I should have paid attention to - the other boats and dark clouds rolling in. AND the wind was coming from a different direction than usual - from dead west instead of the usual north. (Today was REALLY windy with gusts to 35 kn, so I guess a front was coming through. Needless to say, I didn't go out.)
The wind was picking up so I decided to do a granny-gybe instead of the regulation version. As I moved into a full beam reach, and the gust hit. The main wasn't cleated and I let it out ASAP. As I was on edge I was somewhat disoriented with the suddenness of the gust, but I figured out that I needed to get to a run, which I did, but by then I was very close to shore. In the confusion I think I gybed anyway, the accidental version, which changed the boat conditions again. I was able to get the boat point into the wind but had lost momentum and was being blown backwards at a rapid pace so I fired up the motor and powered into the wind.
So:
Peter's note about the bolt rope was exactly how it played out. I had to pull the main down with two hands.
I was wearing my PFD. Had my goods in a dry bag. The lake is warm and it was a warm day, but then again I've not had to stay in for the length of time it might take to get a DS upright and bailed. And we do sail into October. I've only sailed in the ocean strait a few times, and didn't go far from shore and stayed in protected areas on calm days.
I have a large bailer, but need to attach it to the mast. That's an excellent point. It's no good if it has floated away The battery-powered or manual bailer sound like good ideas.
I have a topping lift, which was a big help.
I have end-boom sheeting with a ratcheting swivel-block. There is a cam cleat which I use while trimming, but I always sail with the sheet uncleated and in-hand.
I need to work on "heaving to". I've tried a few times but must be missing something. Though initially it wouldn't have been an option as I think I would been pushed back into the docks on shore quite quickly.
KC's suggestion of a more powerful vang would be an easy upgrade, as would hiking straps.
Reef points - that will be for the off-season. A friend suggested only points at the "second" reef, saying if you're going to reef, why not "really" reef it. He's an ocean sailor, and has been through some real weather.
Ballast is a good idea too, if I insist on single-handing. Which I like to do
I may well try a controlled capsize, just to see how things work out. Knowledge is power. I have to admit I was a little spooked, and afterwards realized that it was because even with the flotation work I've done, I still don't know how the thing will react. I was thinking "Oh, this is going to suck!" LOL. Then, like Brad said, all the noise and scrambling just added to the effect.
I'm not sure that my rigging setup is correct. I still seem to have more sail than I should in the belly, and I can't really flatten it. The sails are "new to me" Jotz, nice and crisp. I don't think my mast bend is correct, which probably contributes to the sail being too full and overpowered. My mast does not look like the one on the North site – not sure it will since I have an older less-bendy profile – but I think I can make some adjustments there.
Thanks again for all the suggestions and observations.