Sounds very familiar, thepterodactyl. As far as counting as depression (I'm pretty big on determining what (for myself) "counts", too.), I'd say yes. After all, you can use words like "mild", "moderate", "severe" etc. to dial in the description of what you're experiencing. They're all depression, but they aren't the same. Like what you say about feeling it tricking you into thinking it might be your baseline, I relate. I have a tendency to discount lesser degrees. Severe is obvious. The others often confuse me. Especially, like you say, when the severity shifts around for no discernible reason. Extra confusing.

One thing you mention I especially relate to. The feeling of overwhelm. It's a pretty good indicator for me, because it's so in-my-face. The less it takes, the worse I know it is.

Oh. And my upswings tend to be more clear cut. Appearing as irritability can sometimes confuse me though.