Advice needed for train travel seat reservations in Scotland

No, you can't select seats off a chart. The computer allocates depending on stated preference and availability at the time. If booking 6 tickets in one transaction it should select 6 seats basically together.

One of the problems is that on the Edinburgh to Inverness route three totally different train types can be used. For each departure they book for the scheduled type a, b or c. But for lots of reasons there can be short notice substitutions with a different type. If you want to select on that route only then use the daily LNER train at about 4.30pm.

Also trains can end up being in reverse formation- so you think you've booked a forward facing or right hand seat, and it becomes a backward facing left hand seat.

Mallaig to Edinburgh the train reverses direction at Fort William anyway, so forward facing from Mallaig becomes backward from Fort William.
The last leg Glasgow to Edinburgh there are no reserved seats anyway.

bear in mind that you don't have to stick with the reservations, if there are a few unreserved seats that suit the group better you can move, just tell anyone else who comes along later looking for an unreserved seat to use your unwanted seats.