I guess reasons 1-2 would make sense if it was very last minute aka a just found out to be broken seat (although from other posts it sure seems like they are fully aware of seats that aren't working for some time).
Downgauge aircraft would depend on the reason as if it was due to low load that is to me a monetary reason and would be annoying. If original plane is broken for x,y,z reason then sure (but again that's something I would argue should not be a last minute issue). Hopefully this is an extremely rare occurrence and there aren't suddenly more posts that are similar as I'm going to start having anxiety about spending a fair bit of cash on some latitude flights that are eupped