Thanks again, everyone! It has been raining and now snowing and I have the mower stuck under a tarp. I will let you know as soon as I am able to get back out there and continue troubleshooting.
On the plus side, I got the brake to unlock. I used a wrench to tap on the caliper and the bar that activates the caliper. It released and we were able to push the mower into the garage.
Take the wheel off, pull the brake apart, and clean it.
There are 2 little pins in there that are prone to rusting so a jambing brake will be an ongoing problem.
Do not touch the centre bolt, that is for adjusting the brake, just the 2 small ones either side
On the caliper side will be a brake pad, a spacer/ shim and 2 sliding pins ( these are what applies the brake )
On the gearbox side will be just the pad which may or may not have a shim behind it.
While the wheel is off, clean the rust off the axel and apply some grease thinly ( I use marine grease ).
Both wheels should come off every few years to stop them rusting on.
I watched a video of someone taking the caliper off. It looks quite similar to automotive brakes (which I have changed many times), but a bit different. It all makes sense. Am I correct in thinking that there is only one break on one side of the axle? I looked underneath (without lifting it) and it looked like there was only one caliper/brake that stops the whole thing.