Prices and window were clearly set on purpose, starting from the number of minidiscs. The size of the originally-leaked material was actually different, so the final one might be coincidental or they might have “targeted” it with strategic padding to be cute.
You don’t have to be a cynic, this is Radiohead - they literally gave away what could have been their biggest-ever album, and don’t really need any publicity to sell out venues.
The only real coincidence here is the MP3 size equaling the number of discs, but I think it’s possible to target a total size with MP3 encoders so that could even be set on purpose.
This material is only really interesting to hardcore fans, who will jump at the chance very quickly. I bet most sales will happen in the first week, if not the first two days, so there is little to gain from keeping it up longer. By setting an end-date, they also add a bit of urgency for the less-hardcore fans who might be sitting on the fence, using FOMO anxiety.
After those 18 days the music will probably be available through other means anyway..