are any other PVR's in the Humax range up to their capabilities, including using the Custom Firmware ?
No.
the 'apparent' problems can be worked around by not setting recordings that I now know will not work properly
All equipment has some kind of limit on its capability that has to be accommodated by the user. Where we disagree is whether your experience is inside or outside the normal operating envelope.
These are design limitations of the HDR-FOX, and cannot be "fixed":
1. It is not possible to record more than two streams simultaneously, plus the time shift buffer.
2. It is not possible to record from more than two multiplexes simultaneously, including to the time shift buffer.
3. It is not possible to record more than one stream from the same service simultaneously, including to the time shift buffer.
Anything not prohibited by the above design limitations should work.
The next factor is how AR and auto-padding recording modes operate:
1. AR recordings rely on the broadcaster transmitting a programme start flag to trigger the recording, within a tolerance window of the EPG-advertised official start time. In general, AR flags are reliable on the major services, but if a previous live event overruns (or under-runs) too much the HDR-FOX will give up waiting (or miss the start flag) and fail to record. If the broadcaster fails to transmit the correct start flag, the programme will not be recorded. ITV tends to transmit the flag a little late.
There is anecdotal evidence of AR misoperation being improved by a full system reset (restore factory defaults).
2. Auto-padding records to the latest EPG-advertised programme start and end times, plus the user-selected pre- and post-padding, regardless of last-minute schedule changes due to live events. It is, however, subject to the above recording capability limitations due to additional overlaps created by the padding. Post-padding is given priority over pre-padding, but if the overlap is due to consecutive programmes from the same service the intervening padding is dropped altogether.
Where recording of a programme is prevented by the design limitations, it picks up when conditions allow (when the conflicting recording ends, including its post-padding).
If you are certain your examples demonstrate misoperation not excluded by the design limitations, the only conclusion to draw is that something is broken, and the overwhelming likelihood is a tuner module (or the data routing from one particular tuner module). Any other fault should affect both tuners, not just one. However, as you have confirmed both tuners (and their data paths) are working, it is very hard to imagine a fault that could reproduce only the symptoms you report.
Therefore, one has to weigh the balance of probability of an almost inconceivable fault that presents as a broken tuner even though the tuners pass the reception test, as opposed to incorrect user expectation.