After hearing and finding the same generic "solutions" on the internet I finally found it myself. (Thanks Microsoft for making a weird ass unreadable error code, 'b7' like wtf does that even mean?)
No enabling disabling doesn't work, it was already disabled and enabling would throw an error.
No setting path doesn't work either because it just eats the Winre.wim file and fixes nothing.
No, I already had a specially made 1Gb recovery partition with all the appropriate markers applied (which is literally written in my post btw).
Turns out that I had to delete (on my main partition) C:\Windows\System32\Recovery files like "Reagent.xml" and "Reagent_merged.xml", after which the reagenc /enable
command would write a new .xml file and work just fine.