They became G4/G5 in a Guildford route numbering standardisation exercise in April 1985 (all routes being Gx irrespective of operator) while before that there was the Safeguard 'A' and 'B'. Not sure if they were exactly the same routes but they were to Park Barn.
Correct. It was indeed the 219 in the 80s, and in fact right up to the end of May 1993, a few months after Stagecoach took over.
It did indeed run Aldershot-Midhurst on an hourly service prior to deregulation as the 219. At deregulation it was split, the 219 doing Aldershot-Haslemere and the 229 doing Haslemere-Midhurst. I believe this was on efficiency grounds: the 229 became a mostly-self-contained hourly service (but switched to double deckers around school times, which came off the 219) allowing the 219 to mostly interwork with the 268 (the eighties version of the 18, i.e. Haslemere-Whitehill-Aldershot).
It's also the case that it was the 19 way back in A&D days (1969): there's a timetable on Timetable World. It was already Aldershot-Midhurst by then, and interestingly, the departure times in 1969 were identical, at the Midhurst end, to those prevailing immediately before deregulation in 1986.
The 20 has had a number of route number changes over the years (20 in A&D days, 452 in the 80s pre-deregulation, then 20 again, and more recently the 'Kite' for a while) but yes, the same basic service.
Likewise the 34 was the 284 pre-deregulation but yes, same basic route.