Anyone going for the job would probably have to use the night buses (if they still run)
London's night bus network is very much still there and still growing (some are 'N' prefixed night routes that are usually two or more day routes joined together, others are 24 hour routes where the same route is followed at all times.)
Frequency varies - some are as frequent as every 10 minutes (every 5 minutes on Friday and Saturday nights) - every 30 minutes is about the minimum.
Depends where you live how many changes will be needed to get to Waterloo.
And bear in mind that with a few exceptions, the night routes stop short of the GLA boundary, e.g. no night buses to Dartford.
TfL's website has a pretty comprehensive journey planner (you can plan by postcode / address - you don't need to know the bus route you want) - or if you prefer old fashioned timetables, the unofficial but pretty reliable
London Bus Routes website is the place to go.
The night tube (if and when it happens) will almost certainly lead to some reductions in frequency on night services that parallel underground lines (on Friday and Saturday night that is) but the plan is for new suburban services on routes that don't currently have night buses to connect with the night tube. A few Friday and Saturday night services connecting at North Greenwich and Morden did start in the autumn, as the 'go' instruction was given from the bus bit of TfL before the underground bit deferred the night tube...
Night buses are same fare as day buses and oyster-able subject to the same daily capping (I think a 'day' is something like 0300 one day to 0300 the next)
Night buses do not run the night of Xmas Eve into Xmas Day, or night of Boxing Day in to 27 December - day buses generally run until normal 'last bus' on Xmas Eve, 24 hour routes pack up around midnight (this does create the anomaly that some routes that are busy enough to have a 24 hour service finish an hour or more before 'day routes')
Nothing is guaranteed for the future - a future mayor may have different priorities, and bear in mind that the chancellor has said that TfL's central government funding will be withdrawn over the next few years. (although my cynical side suspects that if London elects a mayor that central government likes, then he might be able to 're-negotiate' this...)
It's hard to imagine a complete withdrawal of night buses - the network has grown steadily since the early 80s (at which point it was still pretty much the same as it had been in the early 50s, with many routes still following the long gone tram lines round Victoria Embankment, and almost no buses running on Saturday nights - the Sunday papers went to press early enough for Fleet Street workers to catch the last day buses home.)