Is it?Although the short circuit current with a 6 A protective device is not 6 A but something higher time is also reduced so the likely hood of welding contacts with 6 A is less than with 10 A or 16 A
It may be 'better' in the sense you mean it, but if a 10A or 16A MCB were required for valid reasons then fitting a 6A is not possible.so although it is permitted to have 16 A protective device a 6 A one is far better.
Yes, but the loop impedance of the circuit isn't altered by the protective device you choose to install.As to speed that depends on protective device and the loop impedance and with a MCB if within the limits then it should not make any difference, however although short circuit when found do not know if it went full short circuit straight away?
If it is low enough for a 10A or 16A then that might be 'better' than a loop impedance just low enough for a 6A.
Are you contemplating fitting lower rated MCBs (than the cable will allow) on socket circuits?