Thanks, all for the responses.
Glad to hear the retarder doesn't seem to affect epoxy adhesion. I think I'll use it AND go with a rather dilute wash. I'll definitely do some tests on scrap to get an idea of how to achieve the amount of stain I want, but the large scale application is what poses the trickiest issues (I suspect) so hopefully the belt and suspenders approach will pay off. I'm a few weeks out but will for sure post pictures.