You can definitely use a mason jar with metal lid to brew. I use one, but only use the metal ring & then put cheese cloth over the rest of the top. That way the kefir turns out less fizzy (personal preference).
As far as slowing the process...you can use less grains so you make less kefir with each brew. Also, you can lower the temperature. I have slowed things down occassionally by putting the brew jar in the fridge. I don't know if you can dot his over a long period of time, though. It might affect the grains after a while.