I don't recall recipes where Phenidone is (pre)mixed with Methanol, and this page indicates that Phenidone isn't all that soluble in Methanol anyway. That aside, given that Formaldehyde appears to be unstable in its pure form, I would say that most people use the Formaldehyde-Methanol mix, knowingly or not, so the mix should work as expected. Note that commercial prebleach or stabilizer recipes tend to use the Formaldehyde-Bisulfite adduct instead, while C41, according to PhotoEngineer, doesn't need Formaldehyde at all any more.