You need to move all your off-screen sprites to another layout. I believe they were causing most of the problems.
As you know, in order to spawn objects Construct 2 needs at least 1 "default" instance of each object to be added to one of the layouts. But it should not necessarily be your main game layout. Create a blank layout, call it "Unused" and move all off-screen sprites there.
I also made some changes to your code, see my comments in the event sheet.
I added drag&drop behavior to enemies, try dragging them around, see if everything is working as it should.
https://www.dropbox.com/s/cl5907ze1k2af ... .capx?dl=0
Will check out, thanks a lot.
Blank layout solution is top, cheers. Otherwise I would have tried to make all transparent stuff have 'destroy outside layout' (which would involve lots of clickery)