a) The three capped wires are all by themselves, not connected.
It appears that you have one disconnected cable (top of the diagram), showing phantom voltage when one of the switches is turned on. You can't do anything with this without figuring out where the other end of the cable is.
b) Which is the neutral wire than I can use for the two smart switches?
The only likely usable neutral wire is the white of the black/red/white cable. In that cable, black and red are a modern switch loop and white is the required unused neutral.
c) Why is there a low (approx 20 V) voltage in the white wire in the lower left sheath?
Phantom voltage. That happens with long wires next to other wires, particular if one end of the wire is not connected to anything.
To finish up the explanation:
S2 is a traditional switch loop, where white is used as a hot (preferred) or switched hot (which may be the case here based on your voltage readings). That is not allowed in newer circuits because code now requires a neutral (in order to handle smart switches, among other things), and since neutral must be white (at last in a standard cable), that means black and red get used for hot and switched hot. So S2 has no neutral available.