Low pass filter and diode question
R3 and C1 are a low pass filter and are present to probably debounce the N/O contacts at the input.

D1 and C2 is a kind of controlled latch - when IC1A goes high, C2 will rapidly charge to a logic 1 level and, if IC1A's output goes low, D1 prevents C2 from being disharged i.e. C2's terminal voltage remains high and is slowly discharged via R6. In other words C2/R6 is NOT a low pass filter.

C3 is charged slowly via R7 (470 kohm) when S3-5 are open. If all of S3-5 are closed then D2 and R2 act as a reasonably rapid discharge path for C3.