How many amps can 240v handle-tied breakers provide?
Your calculations look correct, indicating that the circuit is currently oversubscribed. The best solution would be if you could put in separate breakers (and wire to match) for some of the heating units. However, you have to be careful as (a) you already have some double-stuff breakers, so you likely will need to turn more regular breakers into double-stuff to be able to fit in another circuit, (b) once you start adding more circuits, other rules such as AFCI may come into play.

As far as the number of circuits. It is typical (though not the only way to do things) to wire things up Panel -> A -> B -> C. In that case, you would see two sets of wires ("in" and "out" or "line" and "load") on A &amp; B as each one connects one direction to get power and another direction to provide power to the next unit. The simple way to tell (provided nothing is really strange) is to turn off the breaker. If all the units you think are connected to that circuit switch off when you turn off the breaker then they are sharing the circuit.