Thanks for the extensive reply, I'll come back with feedback once I got to test everything.
Quick response wrt the battery:
- I already tested the better 3,7V Lithium battery on the new MySensors setup as well and it runs for a bit more than a week. The one in the picture (old setup) has been running for about half a year (since January).
- When using the 3x LR44 on the old setup it runs for months, with MySensors it runs a bit over a day.