multiple-value-variants is broken due to breakage in 'positional-lambda-2.0' which is apparently yet another system that defines a positional argument syntax. Not sure how I missed it, but yet again it's trying to grovel un-eval'd backquoted expressions.
The common theme among the implementations of these constructions is that they're all naive in their code walking, and incidentally in their treatment of literal symbols that happen to coincide with the chosen syntax for positional args, which is irrelevant but interesting in that all such constructs were apparently designed using worse-is-better style.
* (macroexpand-1 '(plambda `(funcall ,:1 ,:2)))
=> (LAMBDA () `(FUNCALL ,:|1| ,:|2|)) ; wrong
Having looked at the source code for plambda, I'd probably rather just suggest that 'multiple-value-variants' not use the broken plambda. It's awfully obfuscated imho and I don't see any immediate fix.