When the new sleeper rolling stock is being used on all sections of the route how many 92's will be required and how many do GDRF have at the moment also how many 73's will be required and how many units do they have at the moment.
If a 16 coach rake have a loco failure south of Glasgow / Edinburgh, what other locos can couple and ETH the coaches. Also north of the Scottish Central Belt what other locos can connect and ETF the coaches to Aberdeen, Inverness and Fort William.
I get the feeling that it could all end in tears due to present reliability of the present locos or is this not the case.
I think you're about 9 months late with the worries about 92 reliability. They're not perfect - they never will be, nor will any loco - but they're much better these days. Caledonian Sleeper has consistently scored top/high up in the punctuality "charts" in recent months, and that's not been achieved with locos that breakdown halfway.
The 73/9s are having a bad couple of weeks - but largely due to the ill-fortune that whilst 3 were in Brush (2 to receive Dellners plus one having collision damage repaired), another one got wheel-flats and another developed an engine issue. This bad spell aside the 73/9s have been on the whole pretty reliable since early issues.
Also worth noting that as well as the Sleeper being high profile (the Sleeper thread has >5,000 posts for example!) and any issue being blown out of proportion, often issues are readily blamed on the locos when there's more to it. The 73 was roundly blamed for the adhesion issues the other day when the Fort Bill ground to a halt and was cancelled, however the railhead conditions were apparently appalling and any loco would've struggled.
The locos rarely fail mid-journey (particularly the electrics) - it's usually when they're being prepped either end issues are found and they're swapped out - often without passengers even noticing. There's only been one loco failure "mid-journey" I can think of in recent months which was 92014 a few weeks back with a broken windscreen wiper of all things.
In the rare event a train fails mid-journey the same as happens now will happen - the good folk in GBRf control along with colleagues in other companies and Network Rail will work out the best way of resolving a situation. If there's a handy 92 at Crewe, say, they may use that. But equally could be a 66 or 57/3 Thunderbird. The locos will still have 'traditional' couplings as well as the Dellners, so at the least can be hooked up and dragged somewhere (e.g. nearest station) if needed.
Whilst admittedly there are a few more options with the old stock, the idea that there's a handy replacement loco just round the corner isn't the case these days anyway. e.g. when 92014 had its issue the other week and sat down at MK, the nearest/best option was a 90 from Crewe...!
5 Class 92s required per night: one each for 1M11, 1M16, 1S25 and 1S26 and one for the Edinburgh-Carstairs portion. Two ECS locos also needed at Wembley and Polmadie, but these could be other locos if modified with the new couplers. GBRf will ultimately have a pool of 14 92s to resource this.
For the 73s, 4 required each night from a pool of 6 to work the Aberdeen and Fort Bill trains, plus a pair for the Inverness.
12x 92s definitely including 006/020 which will probably not back until early next year (although this will be before the new stock). The other 2 are in the 'planned' category still I believe.
The nominal allocation (excluding ECS locos as you mention) is 6 being the 5 required each night and one to cover exams/repairs (hence 6 in the livery). However, some/all of the others should be able to step-in if required (subject to how many will receive the "CAF modifications" including Dellner couplers). Either way, there will be enough...
One of the "spare" 73/9s could also feasibly work the Glasgow/Polmadie ECS, or the Edinburgh-Carstairs portion for that matter - so added options there.