The mention of the Hindhead area reminded me of a couple of oddities from the late 1990s. One was Woodside Road in Chiddingfold which the evening and Sunday 71 would serve but no main daytime route did! The Sunday 19 was similar serving roads that had no main route as well but the details escape me as I never got to travel on the Sunday version of the route. I recall the turning point for the 19 at Churt involving reversing into a very narrow lane next to an awkwardly shaped old building. I gather that’s not totally unusual though as the 51 in Selsey reversed into Seal Road and I believe still does.
I don’t remember Alder Valley in great detail but the interworking and creativeness of the Stagecoach Hants & Surrey schedules during the 1990s was quite remarkable! It was all over the network too, from the combinations of 18/19/60/71/92 on the Haslemere side (and even then the 19 was heavily linked to the 17 at Aldershot) to the completely mixed up Camberley & Farnborough routes linking with Aldershot on/off routes 49/50/56. Some routes largely stayed on their own such as 1, 4/5, 20/20A but there was still a lot going on. A grand old time!
I do remember a couple of occasions when I observed Stagecoach Hants and Surrey in quite a bit of detail. One was in 1993, shortly after the through 260, later 60, Bognor-Guildford was introduced and one was in the year 2000, just after the 60 and 70 were separated.
I didn't really visit Aldershot much but I do remember at Guildford the patterns were somewhat unpredictable. As well as the A3100 corridor (260, 271, 292 - the old AV numbers were retained for a while) you had the 290 (formerly the 260, Godalming via Compton, renumbered for obvious reasons) and the 16 which was a brand new Guildford-Farnham-Shortheath service, on the opposite half hour to the X64 to Winchester (or was it still the 215 then? I forget).
One issue was that half the 260s were Coastline-operated, forcing them to be self-contained, and that forced a 40-min layover at Guildford. The natural pattern would be for the 260 to interwork with the 271, and for the Hants and Surrey 260s this often happened but the Coastlines could not. This had a knock-on effect whereby the 271 then had to interwork with something else, and so on - luckily plenty of options!
Then the 271 would become the 19 at Haslemere and head over to Aldershot. As you say the 19 then often then worked out as the 17 I think, and I believe the 16 and 17 also interworked at Shortheath so you'd potentially get the 271 becoming the 19, then the 17, then the 16, and the vehicle would wind up back at Guildford after a big circle!
This was also a transitional era, with Nationals mixed with Dennis Darts. The double-deck workings were still solid VRs, if I remember right.
By 2000 the 60 and 70 had become separated which ended the complexity necessary to accommodate Coastline vehicles at Guildford. The 70 often then worked out as a 71, then a 19 to Aldershot, effectively "returning to base", though I think there were exceptions. At Aldershot itself things appeared to be unpredictable, as you say, based on the short visit I made there. Also by this stage the 16 had disappeared while the 46 has been introduced, on the former 290 route and extending through to Aldershot via Elstead. Rarely for the deregulation area this was a brand new route and offered a better service than either NBC or Alder Valley days!