Thanks for the experimental .htaccess.
Currently I’m trying the other approach “making sure the files don't get removed” as you proposed here.
I’ve moved out the AO cache folder from the regular cache folder, to avoid that everything gets deleted by WPSC.
Surely Google’s 404s will be avoided this way, but the problem remains that AO’s cache folder will grow ad infinitum.
So, I thought, I will let it grow, let’s say 4 weeks and than start to delete files older than 3 weeks by hand, once per month. This should still greatly reduce Google’s 404s, no? The rest would be covered by the .htaccess 410 entry as above.
But I’m not sure if removing files by hand will not disturb AO in some way?