"As for that matter" isn't used anymore, as far as I can tell. I think it's basically outdated (at least in AE where I can only find it in writing from the 1800s).
I've been trying to come up with contexts where "for that matter" could mean "dabei," but I can't seem to find any. "For that matter" is usually used when something that is true in one situation also applies to another situation. I don't see how "dabei" would fit with that. Vice-versa, I don't think your "slavery-in-history" example is valid. I find the second sentence jarring, almost a non sequitur. Depending on how your sentence continues, I'd probably say instead: History shows that slavery was accepted as normal. It should be explained, however, that ...
Likewise, in your second example, "in this context" seems a bit wordy/clumsy to me. I don't really think you need to translate it at all, actually. Or maybe something like, "What is lactation and what do you need to know about it? One thing / The first thing you should know is that ...
But again, it all depends how the sentence continues.
