Our API efforts are still very early, so I can't promise that things won't change or break, but since we're using this as the API for the iPhone app, it should be reasonably stable. Certainly let us know if you notice any problems, and we'll try and fix them quickly.
As for the polling frequency, it's now refreshing every 5 minutes but also when a content script running on every Memrise tab detects /cave/waypoint page loading
That sounds like a very good way to do it. Currently, we haven't implemented much by way of throttling or caching for the API, but polling every 5 mins sounds very manageable to me.
Ah, good point - no, we don't have a JSON representation of the healthy/wilting counts across wordlists yet. This has been on our roadmap for a while, but it's harder to do in an efficient way than you'd think, so we're going to have to do some careful denormalization first.
In the meantime, maybe it's simplest to keep things as you're doing them - if we end up rejigging the markup on the dashboard, we'll try and release a JSON representation at the same time, so that you don't have to rewrite your code multiple times.
I know it might not be the best place to ask this, but would it be possible to combine the Language Immersion for Chrome addon with Memrise?
That extension uses Google Translate to translate some words on the websites you're visiting to the target language. I'd much rather have it use my Memrise vocabulary for that, especially for the Chinese characters I'm just starting to learn.