Advice on choosing components for a flyback arrangement, for switching inductive loads fast
I would just place a TVS diode from drain to source of the mosfets, that makes it easier to match to the maximum ratings of the FETs and removes the need of a series diode. 

If power dissipation is an issue, either add the transistor or use multiple TVS diodes in series with resistors (see below) in parallel. 

Something that you might not have considered is power sharing among MOSFEts. Depending on where you are in their gm/temperature curve you could enter a positive-feedback regime in which the hottest one takes an increasing portion of the current. This quickly leads to a cascade of failures that destroys all of them. 

A small added source resistance can remove this possibility. The same is true for multiple TVSs, the same resistance can work for both.







simulate this circuit &ndash; Schematic created using CircuitLab

But. As you are talking of tens of microseconds you would need to let the voltage build to ~1kV. This would require separating the protection of the MOSFEts from the voltage build-up on the inductor, which is not easy to do. It is much more practical to simply increase the maximum drain-gate voltage of the existing Mosfets (>800V Mosfets are not that uncommon nor terribly expensive) 

An example of how to isolate the drain of the mosfets (which requires quite a few extra components and an auxiliary winding in the solenoid to simplify their gate drive), is shown in the following schematic. Note that this is not a practical circuit, as (1) it will clearly cause breakdown on the gate-source junction of the PFETs, and (2) it more than doubles the number of required components, as it places them in series with the main current.







simulate this circuit

A more practical circuit, but still greatly increasing the number of components is below. Note that the gate drive resistors are taking a considerable portion of the power dissipation:







simulate this circuit