The memory protect switch in Sys 6 and later does not write protect the whole CMOS RAM, only the area that contains settings. Otherwise the bookkeeping data could not be written to cmos. If you have problems with cmos data corruption, adding a memory protect switch would not help.

You should check the part of MPU circuitry that disables CMOS RAM (or the NVRAM in your case) when supply voltage drops. Since you dont have original boards, I cant tell how it works in your board.