@Nawrot: That’s great and all, agreed. I never use LB myself for the reasons you’ve stated. LB could/should not exist for all I care.
However, this is not stack overflow (obviously 
where an answer to a question “how to do it” is “don’t do it”. We cook spaghetti script here.
The question is how to do it - how to hook up a dynamically spawned actor’s dispatcher call to a Custom Event in the LB.
Not whether it’s worth doing or being done right. It isn’t, and OP knows it, admitting it being a nightmare.
edit: at @OP - refactor if it’s no too late, avoid LB - unless you have that one self-contained level with unique functionality that matters in that one level only.