Hello @Pieter-Blommaert see my replies below:

what is the best way to detect that the boiler and the thermostat are online you could use the statistical registers, eg. 2004 and 2006 (or 2104, 2106) does it make sense to switch the gateway between monitor and active mode (e.g. in normal condition, i rely on the thermostat, but in case the regular scedule needs to be overruled, i switch to active mode (with the thermostat still connected) of course, it is only about the coil configuration of the coil whether the gateway is repeating the communication to the boiler from thermostat or no... Experience in steering opentherm boilers, do's and dont's... cannot tell much here, but definitely all pros! :) Neuron/Neuron software has a higher level interface where i could integrate simply and just set measured and expected temperatures? (Can always consider to switch to Neuron, after all including heating control is a rather advanced actually all you need is to set the boiler on or off and set the expected setpoint. The logic of those has to be done in your application. In Mervis there are function block like equithermal curve and time scheduler to help you write your own "thermostat"