if I have London Terminals to Inverness super off peak route any permitted is it OK Euston - Glasgow Central then Central or Queen Street to Haymarket or Edinburgh then on to Inverness on a direct service?
Yes, that route is valid, provided you don't double back anywhere (other than at Group stations such as Haymarket-Edinburgh). So for example you couldn't take the LNER service from Edinburgh to Inverness if you'd gone from Glasgow to Edinburgh via Polmont (the way the quarter-hourly on-the-hour service goes), as the LNER service also goes via Polmont. Of course that's in theory - in practice, it's difficult to enforce something like that, but I would still try to avoid doubling back.
If you're interested in the technical details, the relevant
map combinations are LM+EG+EI and LM+EG+KI.