Little bit of success now after some further testing. I've narrowed it down to not being caused by TFM 412.
It seems to be something to do with one of the following -
SAS Big clouds by Manysh
SAS Blender Waves
SAS Cowling Engines and Props
SAS CTO mod
SAS Forgotten countries
SAS FOV mod
SAS Small FX collection
SAS German gunpods
SAS Gore for all planes
SAS guncam tracers
SAS New mod wheels
SAS Random belt start pos
SAS Water-4 by carsmaster
I disabled all those, and I now have mission pro combo and flyable AI aircraft after all the TFM 412 packs and the 633 squadron intro now works when the game is run as SAS Modact.
Obviously now I have to go through the process of finding out which of the above is the cause of the problem, but that means disabling all the TFM packs and enabling them again, which as stated is a 7hr job!!
I don't suppose there's any chance I can get away with having any of the above after the TFM 412 packs and not run into further problems?
EDIT - SOLVED IT!!
I narrowed it down to just one of the above mentioned optional SAS Modact mods - #SAS_Water-4 by Carsmaster.
If I have that mod enabled the intro for the 633 Squadron doesn't work (nor do any other addon campaigns intros). If I have that mod disabled the intros do work. At least for 633 Squadron and one other addon campaign I've tried.
Don't know how to modify that particular mod to fix the problem so the mod is usable, but at least I've solved the problem (for the moment at least) by disabling that mod.
Guess I'll be leaving my PC on overnight now though so I can reinstall all the mods in the right order (the TFM 412 thread says to install the #SAS mods first and I've now got them last in my list!). Oh well...
Hope the info I've given here helps someone else out.