Anne Miller wrote:I am fairly certain that if it has not already been mentioned that the solution is a Rocket Mass Heater aka RMH.
Yes, RMH has absolutely already come up earlier in the discussion. While RMH is undeniably a superior heating technology, especially for heating when fuel is scarce, the problem is that the OP is expecting to spend this winter in a rental property. They will not be at liberty to make permanent modifications to the building, like installing an RMH, nor has the OP expressed willingness to make a substantial capital investment into property that they don't own.
In addition, as has also already been discussed, we on permies like to tout anecdotes of RMH being used as "sneaky heat," but so far as I am aware those are all based on examples from the USA, specifically I believe in PNW jurisdictions with wood smoke bans. The successful "guerrilla" installations of RMH in those anecdotal accounts likely did not have to contend with the level of pro-active code enforcement that is being reported as routine in Germany.