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So it is 3pF? Is there such a capacitor?
Nope, that'd be comparable to the capacity of a wire !
You read it backwards, it's 18nF.
I think you forgot on your shematic that pins 1 and 28 of WRAM are connected together (WRAM-VCC).
D2 and D1 serves to switch the correct supply to WRAM : VCC when the game is turned on, and the battery when the game is turned off. R2 is here to artificially increase the internal resistance of the battery, very likely, to prevent the battery from powering the circuit at all when turned on (the WRAM chip draws a lot more current than when idle).
CB or whathever name it is, is here to prevent sharp voltage transitions on WRAM-VCC. When the power is turned off, the voltage will slowly decrease from 5V to 3V instead of doing a sharp edge. Earlier MMC1 boards lacked this cap, the saves were lost more frequently (unless it was something else ?).
R1 is here to disable the WRAM when the power is off. I'm not sure what R3 does. It seems to attemps to force the address like A14 down (out of the $6000-$7FFF region) when power would be half-on, on enough to have the CPU doing ghost writes, but not on enough to drive the address lines properly. I wonder if this had any effect at all.