So what can be the worst of all evils
Juggling with changing the MPU to avoid the bugs has gotten me back pretty well where I started but here are some of the alternatives and why I dismissed them
1/ STM32F303 I was so impressed by the data sheet I went ahead and bought one, it was only in the latter stages of evaluation I realised a showstopper, the PWM although it did everything else did not support half-bridge or push-pull SMPS's and neither did any of there chips! Turns out there PWM is a very common piece of ARM related IP used by many other ARM MPU vendors so wiped all of them out too including the often mentioned NXP.
2/ Cypress PSOC5 Again very good spec and no worries regarding PWM weirdness as incorperates programmable logic where a custom design can be implimented. First indicator of problems was no errata on the website and when tech support contacted said no such document existed!! THEN I downloaded the IDE, OMG like a childs toy plaything, you are supposed to draw schematics of the design then it goes off and adds whatever boilerplate code it thinks is appropriate, the final straw came when I finally found the RTL description in a seperate document to the functional description with a flat index 45 pages long not even alphabetical, upshot, unuseable IDE and no alternatives (I asked).
3/ PLD was possible except all analogue peripherals would have to be external, but the killer was development system cost.
4/ A simpler PIC that I have used in the past with well known PWM's. Sadly re-discovered why I had chosen the PIC24EPxxxMCxxx in the first place as the PWM is an advance on all those preceding it.
It is very difficult to fix power mosfet drive problems with external logic as it must be secure against mis-operation in all power up/down circumstances. In the end I have decided to persevere with the PIC24EP as the least of all evils
and I am at least now able to quantify some of those even though Microchip show no interest at all. For the time being I think I have devised a discreet circuit with known power cycle parameters that will work-around the PIC problem that was causing MOSFET loss, so the parts are on order and I hope I can replace the lost power mosfet 2nd time around without damage to the pcb, this being a hard ask on a high voltage part. In the meantime there is software to re-write to accomodate the changes and steer clear of the bugs
I hope my brief assesment of some alternatives might help others