frawjōn

The subjunctive didn't have a unified verb plural, but it did lack the -t in the third-person plural. You also have to consider that Middle Dutch lost the -t in the indicative, so there's no reason why certain Old Dutch dialects weren't already in the process of losing it, too. It's not like the -t disappeared right at the moment we begin to call the language Middle Dutch, it could have been earlier or later.

Old Dutch didn't completely lack the nasal-spirant law, but it was carried through far less thoroughly than in Old Saxon or Old Frisian. Other examples are vijf and zacht. But note that 'Muiden' and related words all occur along the coast, an area which was historically Ingvaeonic-speaking, so it's natural that the influence of "Ingvaeonisms" was stronger there. The Frisians are known to have lived along the coast of most of Holland during the Roman age (in particular what is now still called Westfriesland), and only later did the Frankish tribes from the east move in. Old High German wasn't entirely free of Ingvaeonisms either, one still survives: Süd.

No comments about zacht, but vijf and Süd are somewhat irregular.

vijf

foif#Alemannic German isn't really "North Sea Germanic", I suppose. Earlier Appendix:Proto-Germanic/fimf mentions "an irregular consonant change" from

Appendix:Proto-Indo-European/pénkʷe, which gives

• Appendix:Proto-Celtic/kʷinkʷe, which gives
  • Gaulish: pempe
  • Brythonic:
    • Breton: pemp
    • Welsh: pump
• opposed to
  • Goidelic: *kwe(n)kwe
• and Italic, which gives
  • Oscan: pompe
  • Umbrian: pumpe
• opposed to
  • Latin: quinque#Latin

I think Germanic:fimf, Celtic:pempe/pemp/pump, Italic:pompe/pumpe differ in a visibly obvious way from other Indo-European languages.

Süd

I would almost dare to say that sunt#Old High German is the odd man out. We have suðr#Old Norse (a bit early for Low German influence) and Romance languanges often have sud/sur (even sud#Romanian)