After some digging, I now have some answers to my original questions. Yes, ATC in the N560 airway south of Inverness is delegated to Inverness (up to FL130), hence departing traffic being seen still wearing Inverness squawks when at FL100+ some way south of Inverness. This is spelled out in the NATS/HIAL/BGA LoA on the "GUSSI box", which allows gliders to operate at weekends in a 3 x 15nm chunk of N560, up to FL190, without needing a clearance to enter and without a transponder. It has to be activated through a request by Feshie to Scottish.
What is slightly peculiar about the GUSSI box arrangements is that "IFR aircraft are not restricted from entering the GUSSI Box but should not be encouraged to do so" and "Controllers are not required to provide navigational assistance to pilots to avoid the GUSSI Box unless requested" - a little odd given that all IFR traffic in this airspace will be under a radar control service.
I also find it worrying that, given that controllers are not expected to actively vector traffic away from the box, so the onus is on non-glider pilots to know that the GUSSI box is activated and where it is, the LoA states: "The GUSSI Box is promulgated in the UK AIP". I can find no mention of the GUSSI box anywhere in the AIP or AICs. So how do pilots know?
Coming back to some of your points airwave:
I'm not usually any good at maintaining either a level or a heading and don't usually try
Crossing of/operating in N560 without a squawk (and therefore requiring a clearance to operate non-SSR) would not AIUI require maintenance of a heading or a level. You'd be given a "not above" clearance and asked for an expected exit time. The only thing it might stop you doing is climbing.
Depends entirely on the type of service being provided to the IFR traffic as class E isn't really controlled airspace
I know you really want that to be the case airwave but unfortunately it IS really controlled airspace, with all IFR traffic on a radar control service. There's no option for IFR traffic to be getting anything other than an RCS.
Coming back to the GUSSI box, since this only covers the eastern 3nm section of the 10nm wide airway, it doesn't provide a mechanism for gliders to cross the entirety of N560 other than by (a) having a transponder or (b) getting a clearance from Scottish.
Slightly irked that the FL100 restrictions only apply to Scotland (must have transponder above FL100 or prior Notam) but not England and Wales
AIUI the requirement for NOTAM activation of the Scottish non-SSR area only came in when the exemption was re-negotiated in April. However, bizarrely, the new version of the exemption - ORS4 No.1162 - contains nothing about requiring NOTAM activation. So it's not clear how the published rules prevent anyone from flying non-SSR over Scotland without any prior notification.
As regards them being different from the rules in England and Wales, is this not just a case of the Scottish area being first up for renewal, and NATS/CAA will seek to apply the same provisions when they review the exemptions south of the border next year? It may also be relevant that the Scottish exemption applies to virtually the whole of Scottish airspace, whereas the other areas are much smaller.
NS