Am I correct in saying only enemy bases can spawn elerium? The only time I see them is when they have extractors on them.

That's right. The extractor will create more Elerium, if not destroyed. But there is a limit. The larger the amount, the slower it will accumulate.

so enemy extractors spawn elerium. I suppose our extractor will deplete it to nothing? In which case we need the enemy bases. hmm... Can I ask also? I seem to be getting random explosions in my base. Please tell me you havent got tunnelling banelings?

I think you're attacked with acid bombs.