The "slightly baggy MTB shorts" are actually a double-short: the inner short is a liner with a chamois to help prevent chafing (they're meant to be worn without underwear, btw). It's a pretty good solution for that temperature range; however, I don't know anywhere in Wisconsin where it's 75 degrees year round. The liner of those shorts is detachable, so if it's got a decent chamois, you may want to experiment with wearing it under other pants as well.

The statement that "SPDs aren't really useful for a commute" is marginally true if one believes (erroneously) that usefulness is only measured in terms of a "performance gain", and nonsense otherwise. If you are using clipless now and like them, that's what you should use for your commute.