Ok, so I've just done the above procedure provided by Wirby and can confirm that this resolves my problem of dead node for my first module, so maybe it should advised to do an auto inclusion rather than a manual one with this module.
With auto inclusion, I can confirm that there are 4 detected devices all with "On, Level 255%" type under devices in domoticz tab with IDs going from 00 to 03.
Which one is the good dimmer that control the 6 states (I would bet on the ID01) ? and which 3 ones are the one that are the switch I1/I2/I3 (not cabled on my case) ?
Wirby, could you please check your ID's and tell which one is the real Dimmer which control the 6 states of the heater please ? So if another guy is searching for the information, he will be able to find the complete procedure to add the module to domoticz with this thread

Thanks again Wirby for your help and time !
Regards,
Laurent