My whole class was high on cocaine and we started to attack elder men in corporate suits like wild animals (remember the mall scene in Mean Girls? similar to that).
Do you think if a controller gets high does the yeerk also get to enjoy the high
I'm going to say yes, if for no other reason than Edriss's smug confidence that (it's implied) she's keeping Jenny Lines quiet with cocaine, and the contrast to her being way too invested in humanity. I don't think we see yeerks drunk, but we do see that poisoning a host will at least temporarily poison the yeerk (e.g. #21). So I think that yeerks can get high, and tbh I don't think they're protected by the blood-brain barrier.
That said, I love the headcanon that yeerks are confidently wrong about that. Sort of the way that for decades psychiatrists were like "of course I am just a neutral observer, I do nothing whatsoever to influence my clients" until ginormous heaps of evidence started piling up that psychiatrists can accidentally give clients memories or even entire mental illnesses. It's easy to assume that you're superior and just an observer, even in the face of contradictory evidence, and I think the yeerks would have that assumption.