If you know many other people in your situation who have navigated this situation before then they would be the best to advise you.
As I see and I can only go what you have posted so not sure if that is all correct.
You are in Germany and have no immediate plans to relocate back to the US, one of the common reasons for exceptional circumstances.
Your wife is in some unspecified third Country and cannot enter Germany for reasons that are unclear.
You wish to process the I 130 in Frankfurt so she can obtain an Immigrant Visa to enter the US, then obtain a GC, well more immediately a I 551 stamp, which will then allow her to enter Germany. I do not know if Germany requires the Card or the Stamp but that is just a matter of timing.
I see 2 issues, the one that seems key is how will she attend the interview in Germany? The other is persuading Frankfurt to handle the case. But if she cannot enter Germany anyway, well.
How did the people you know handle this?
Presumably she can interview in her home country.