As far as i know the ocs can't turn back to aga without reboot. If you boot with aga yes you can use ''killaga''
commands from the shell (if you have any in your c: directory, if not, you can search at aminet for this. With some shell commands you can turn on and of the cache of the cpu after the boot. I never use the WHDload and i don't know how it works but if you have a setup that runs ok on the a600 go and copy it to the a1200! probably will work. As for the aga game,maybe it needs ram... try to find at least a ram expansion or even better a 68030 accelarator to gain more cpu power together with more ram