Hello Gary.
The gate of each MOSFET is protected ,I think , by a zener diode ( 12 V ) on your scheme...so the power supply can climb at 48 V probably , "full mouth" of limit intensity of the MOSFET.
I think we can put 2 MOSFET in paralel in each branch , but certainly it wil be necessary to adjust the gates résistors ( down) and the zener .
Have you try to add some turns ( 1 or 2 ) to the heat coil ,and remove 1 or 2 capacitors in the LC circuit...this change make a frequency not change very much , but certainly a bade sinsoïdal signal ( quasilly square) .
But the efficiency would be better I think , and this pseudo square wave make more saturation effect in metals ( more induction) .
To verify...of course .
For the U shape problem ,... for me ,...you have a frequency of 150 kHz with a hight power , so a wire curving on left or on right in a branch change the impedance of the circuit, because of HF induction 10, 20,30cm around the heat coil.
So a protection by a metal box of all components exepted the heat coil , seems to me a good approch .
Have a good day.
What is the target of your prototype ...?
Jac.