The ADC inputs cross-talk via the capacitance of the ADC input, about 8pF, which gets switched around
and thus dumps charge between inputs (only noticeable for high impedance inputs). There is basically
no DC leakage (that 100M is a worst case figure at full temperature range of the chip, typically its way
higher).
You shouldn't put a large capacitor directly on the output of an opamp (unless the datasheet allows it),
as stability could be lost and it oscillates.