I suspect point 1 happens every time as it's the only viable option. There's some pretty resourceful folk working in Sleeper Control - they need to be - so if there was a way that worked within all the constraints they have to deal with I suspect it would've been found!
As
@najaB says, though, the various portions take up valuable platform space in Waverley and cannot stay there - also no point attaching the portions together when they need to be separated to head back north.
Whilst bad for the impacted passengers getting rudely awoken and turfed off at c.3am; if the three portions are not sent back north in a situation like Mon night where the Northbound service was cancelled then there would be no stock/locos at the northern termini for the following night's service and there would be another trainload of passengers impacted.
It's also not a simple case of parking them up somewhere and send the empty stock later - there's only so many drivers/shunters/crew available, with hours they need to work within; only so many paths; only so many places 16 coaches and 3/4 locos can hang around without causing a lot of issues. There's also the fact the crew need their required time-off until the next service runs, and the depots/stabling points need the requisite time to service/prepare the stock and locos... I suspect there's a further long list of factors/obstacles that have to be considered on top of this.
Looking at all the other services on Monday night three trains (1S26, 1M11 and 1C11) were kept in stations overnight to be used as 'hotels' - and for a fourth (1S25) the loco was pretty much single-handedly detached, run round and reattached at Warrington in the dark on the WCML Fasts (no mean feat) to get the train back to Euston for passengers to finish off the night there. Suggests CS/GBRf do what they can to maintain passenger comfort/sleep - and the situation in Waverley is because there isn't really a
feasible alternative. Appreciate this is small comfort for the sleep-deprived passengers on the southbound Highlander.