I agree with you as to what happened in Caversham, clearly Caversham Bridge was closed around that time. I am sure that when you get going the wrong way in a bus it is not easy to just turn round, and the traffic was probably awful too. After that, given that the vehicle tracked going places at times which fit together, I think one can be confident that it went the way BusTimes says it went. Sometimes tracking is patchy, here it was consistent. It did not divert through Twyford and headed straight to the A404, I am sure it would have been empty. As for why it diverted before reaching Handy Cross, sometimes traffic up the A404 can be quite bad and it can be safer to divert this way. It didn't go into Marlow though.Is this purely from bustimes, purely from observation using bustimes as a back-up, or a mixture of both?
There were roadworks in Caversham on the day you quote (at least according to local news and council websites, and Reading Buses), starting at approx. 7pm. It seems as though the driver of that vehicle got a bit lost - perhaps confused because they weren't expecting them, being due there before 7pm? Perhaps due to having to follow inadequate guidance (trying to divert using Google maps on a phone and not break the law at the same time? Perhaps because of vehicles setting up the roadworks meaning some turns were inaccessible? - but made it to Reading eventually. Not sure you can tell for certain that the 850 journey was "clearly running empty" either - as it left 75 minutes late, it may have only carried a few infill passengers and run fast where appropriate. Otherwise, why run through Marlow, rather than sticking with the A404 as far as Handy Cross?
There are oddities with the tracking, Streetlite 2325 in particular. Often it tracks for a journey or two then vanishes, not sure if has really stopped or just stopped tracking. Yesterday it tracked from 06.22 until 06.27 but it may have continued all day as nothing else went on its duty. Today it has tracked just between 10.29 and 10.33 and again nothing else has been on its board. Some of the Versas are quite flaky too, 2957 appears to have given up at 6.18 yesterday and 7.47 today but not sure it really has. One - 2979 - appears to have gone off fleet (it hadn't been out for ages) and there is a spare ticket machine operating, but this still doesn't leave enough vehicles given that nothing gets any break between the peaks. I suspect maintenance is minimum legal plus breakdowns only, no chance for anything preventative.