Trying again with a new calculation path, the first attempt was obviously wrong:
I’ll report back on how this behaves
The last setup gave the following result:
the kWh Accumulator starts at an offset of 8.9kWh and accumulates hours values monotonically, as would be expected, but unfortunately not what I am after… And (also as expected), only hourly values make any sense, so for now it seems indeed that I will need to process the kWh feed in Home Assistant directly. Getting the feed into HA is already in place. And fortunately, there are a number of people that have been doing trend data processing and calculation of interval data, it seems.
I found a solution in Home Assistant - Utility meter. HA has possibility of creating periodic statistics from quarter of an hour, hour, daily, weekly etc. For my use, it takes the kWh feed as input using the emoncms API-key.