Ok I tried just item 1 in above list (just cleared both cache and storage for Web Calendar App, and forced it to stop), then I went back the /e/murena account manager, went to the third tab “Webcal / old agenda smth”, three dots → sync, then activated the working subs I want
Now it works, no need to delete the /e/murena account, now the crazy broken ghost thunderbird hols webcal is gone, no more notifications !
Getting this vague feeling there is a message about how difficult it is to get holidays 
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(btw the webcal app it is forked from is ICSx5, the DAVx5 is the one behind the /e/ account manager)