Is spiritual adult-ery (something adults do) whoring with Law after claiming to be espoused to Grace?

I think I get what you're saying. If we are already saved, then we don't need to worry about being "good enough". We just need to repent and make sure that our minds and hearts are in the right place. Memorizing and following a big list of traditions isn't what saves us. Is that what you're saying? Yes, legalism could be considered a form of spiritual adultery. I've never heard it put that way before but it makes sense.