Got it --talk about esoteric! Days ago I had set fingerprint protection within my ScriptSafe extension, and I had enabled "Prevent Clipboard Interference." That's what was stopping me from accessing settings and themes within my Gmail account. I've now whitelisted Gmail in this one specific area of my ScriptSafe extension , and all is fine now. Finding that was finding a needle in a haystack, but that was the issue.
Thank you, mac and PCNetSpec. This topic is solved!