>Where does the chip get its timing if there are no external clocks and the internal one is disabled?

nowhere... if it's working, let me know how you do that!!! Or maybe you programmer don't set fuses correctly or some aliens do the job for you!!!

>So what "clock" is working with MCLR=ON?

depending of you setting INTOSC, RC, XT,HS.... As already said... MCLR is only Master Clear, acting as an RESET.

regards