I think I have seen enough. My DHO1074 is now a 70 MHz version with basic memory again, with fans running at full speed and no Key.data file just like I received it. Wiped the fingerprints off the screen, put it back into its box, and triggered the return process at Amazon.
There is a lot I liked about the scope. The 12-bit front end is "real", with low noise and usable 14- and 16-bit modes. The screen is brilliant, although a bit too reflective for my taste; the UI, while maybe a bit playful, works quite nicely indeed. And the decoding and measurement functions do work from full memory; I had them count 153000 pulses in a region defined by the on-screen cursors for example.
But there are just too many rough edges and cut corners: Measurements can't be changed after being set up (or at least not all parameters?); the Flex-knobs change parameters on awkward scales and get confused as to what they are controlling; FFT resolution and window functions remain dubious; recording mode is lame to an extent I can't understand...
Using the scope always felt like I was tip-toeing, weary of the next little stumbling block. Since this series has been in the market for more than a year, I don't have very high hopes regarding firmware improvements. And Rigol Europe, who took four weeks and three reminders to finally send me an upgrade code -- which then did not work at all -- did not instill a lot of confidence in the customer support I could expect.
Kudos to Amazon for their generous return policy. Their 2-month return window allowed me to be a good boy and wait until Christmas, then evaluate the scope in some detail. I keenly await Siglent's price for the SDS1000X HD, and will probably shell out the extra money for that scope when it eventually becomes available. In the meantime it's back to the DS1054Z, to re-calibrate my expectations.