Seeking marriage proof

Grandparents were married in Diwitten 27 Sep 1937, catholic I’ve contacted the archives of Allenstein and several local churches however, I believe the marriage records were destroyed in the local church and somehow it’s not available in the archives in Poland or Berlin. No records in Arolsen.

What other forms of proof are accepted in modern day Germany for proof of children born in wedlock?

Any suggestion in tracking down documents of displace persons who were in east Prussia prior to 1945 & then emigrated to US in 1951? Not much of a paper trail.

I do have US naturalization records but only family Reisepass and photocopy of familienbuch.

I appreciate any assistance or suggestions