That’s is sad to hear. I really like this game and I wanted to see it finished. Personally I am working on a similar project and I always like to see that there are at least one or two competitors that are making up similar games but probably with totally different approaches for the engines. Yours looks awesome and fluid with a lot of nice concepts.
In my engine I have similar problems like you have. The number of enemies in the level is an issue and also the number of enemies visible at the same time is an issue (at least in my engine). On the last point I am currently working. The number of enemies I have not yet concentrated on.
If it helps, I’ve had the idea of a kind of an enemy plug. This means you have a slim data structure that has e.g. x,y,index,flags (4bytes) and these kind of “enemies” you can spread in huge numbers over your map. Then in the back you have like 10 fat data structures that actually cover the more complicated and memory consuming parts for a sprite. The slim data structure points to a fat one by “index”. It is just a first idea and probably there are problems which I have not thought of yet. This of course limits the number of visible enemies and also causes the state of some enemies to start over again but it may saves RAM.
So for the sake of competition and because I really like to see this game come to life I hope you will continue your efforts to make this game happen