@GotnOGuts as always, thanks for being that person who responds when other people keep silent 
Going with storing the file(s) before Camunda is the main problem - “people wanted” to have the files submitted (and by magic) processed by Camunda. This is basically “part 2” of the topic, where I was looking for a way of proper file submission. I do realize all the majority of problems sending files as regular variables poses, but “not my decision to make, sadly”.
I was hoping that by editing the variable BEFORE it gets submitted (as was suggested by this fine person here - Listener for form submit - #12 by Draakzward), the original variable doesn’t get persisted, so all should be fine. Turns out that place is “too late”.
Alternatively, I thought of “smashing in” my own FormServiceWithBlackJackAndLadies, but that approach, although possible, bricks the DI logic creating cross links and thus crashing the app. Also need to spend time trying to reproduce the precise core Camunda config (can’t just @Bean the FormServiceImpl2). So that’s currently out of the scope of possibilities.
And I don’t see a way of actually removing that variable. Not without doing a native query, but I already sense that I will be reminded of my sins somewhere in the future.