So....
Tried to reinstall SH4 under WIN10, and tried all above for a day.
No joy.
Utterly frustrated, decided its another time, and got UBOAT on steam sale.
UBOAT didnt ignite with me yet, reverted to boardgames.
Had to update IRFANVIEW program to scan a depth gauge to print, and clicked SH4 exe for the heck of it as I scrolled by....
It launched!!!
So, FWIW, IRFANVIEW seems to get me back...
Edit: now SH5 also works again...