It took about three hours of trial and error, which I did on a Friday so that the lights could be out without bothering Siobhan (and while I could still see by sunlight), but I finally was able to remove the Z-Wave switches that I'd had installed to control my living room lights, and put the manual switches back in. Along the way I learned a lot about four-way switch wiring and how this particular circuit is wired, and I suspect that if I felt confident some four-way switch system would work, I could probably install it myself this time.
Of course I'd have to be pretty confident. Given that, after almost two weeks, ACT Solutions had not even responded to my email, I think I can safely say none of their products are going to be on the list to be considered. I'm not sure if other manufacturers are making a switch set like this, and if so, if they do better in either the quality assurance or customer service venues. I'm going to let this whole situation rest a while before I even consider it, though.
Figuring out the circuit was a bit difficult because there are two different ways a four-way circuit can be wired. But it turned out in the end that the difference is not significant for what I was doing. It would be for putting in the Z-Wave switches because of them needing power but not for the mechanical switches.
It turns out one of the biggest problem I had was figuring out which contact on the switch was which. The diagrams I was going on suggested they'd be in particular places physically on the switches but that was deceptive, and ultimately I needed to use an ohmmeter to figure out the internal wiring of the switch. That's the kind of thing any electrician would take so for granted they wouldn't even realize a consumer might not know, or might not even realize they didn't know.
I had a similarly obvious problem with the connections. These switches have screw terminals and also have a slot into which you can insert the bare end of the wire. It seems that using the slot is not that reliable. I know that electricians rarely use them, but I wasn't sure if they had a good reason for it or if it's just tradition. It turned out at least with these switches it's for a good reason; even with the screws tightened the slots weren't giving a good contact. I probably had the right configuration once or twice and dismissed it because something wasn't making contact somewhere.
When it finally worked, though, I was exultant and triumphant. It's the kind of job that would take an electrician ten minutes, and took me three hours, but I did it, by myself, without even any advice (not that I ever get useful advice when I ask for it about this sort of thing). And I learned a few things along the way.
Unfortunately I have had no such luck on the floodlight. Odds are I could replace either the lamp itself, or the motion sensor, though it wouldn't be easy for me to figure it all out -- particularly while way up on a ladder. But I am not sure how to tell which is the problem. I think I'll just let this whole problem sit until spring when it'll be easier to work on it, and then see if I can open things up and figure out which part is in need of replacement.