Possibly Rogowski coils. You don’t have many breakers in the panel so another alternative would be to measure all of the loads and add them up for the total.
It’s OK to use one CT if there is no neutral.
Practically speaking no.
Never actually tested for extreme temperature. I expect it would work OK.
All meters need a voltage reference. Sense connects directly to 120V and measures it internally. IoTaWatt uses an external transformer and as such can adapt to any voltage with the proper transformer. Most of the world is 230V.
No
No, I don’t know if they will work. Experience suggests that they won’t.