However, I can now say that microwaving the stuff does warm it up. I got a couple of hot spots after 1 minute, but the huge frozen block in the center was still frozen. I shook it a bit and then put it in for another minute. Again, hot spots, and in the same place.
So, I shook it, positioned it facing a different way, and microwaved for another two minutes.
At this point I had melted enough that I figured that, combined with the dregs from the old bottle, I had enough liquid to wash a couple of loads of laundry, and I left the large bottle to melt in the bathtub, in case it had developed a leak.
The melted detergent looked, smelled, poured, and most importantly, washed like detergent ought to, so it appears to have worked out. Also, the large bottle did not seem to leak after being left in the tub for a few hours.
So, there you have it. Again, I do thank you for the reasonable suggestions; if I hadn't been under time pressure I would have waited longer for replies, and actually followed them. :) You're all more sensible than I am.