German Citizen but Dutch Resident wanted to marry Vietnamese Student

Hi,
This situation is a bit complicated.
My girlfriend is currently in the last few months of her masters degree and I am self-employed with my own dutch B.V.
I have lived in Thailand for 5+ years, so the "geemende' told me I need to get an official paper from thailand confirming my single status. I never had a marriage registered i Germany which is the country of my origin.
Is the geemende correct about that?

I have the document from germany that I am free to marry, my birth certificate and so has my fiance.
The document from Thailand is an insane headache and I am biting my teeth out on it.
It's also very time sensitive as we are a couple and living together for 3+ years and her education visa is about to run out once she finishes her masters.

Any advice on how to handle this properly? Should we just try another 'geemende' ?
Once married, what will happen to her visa? Can she simply change it at the Vietnamese Embassy to a marriage visa?

Thanks in advance!!