No, it can only be used to measure a 3-phase consumer or producer. Shelly has its own devices for one phase

Yes, but the app will only show phase A, phase B and phase C. You then have to know which phase is which consumer

Do you mean a second one? Theoretically yes: L1, L2, L3 are analysed separately. Yes, you can use that if that is the most favourable or practical solution for you. Otherwise it is better to use your own devices (see other answer).