Hi,
This is an unfortunate situation and I can understand it is frustrating.
There are apps in AppSource that are "Certified by Microsoft" and there are (the vast majority) of Apps that are published by the vendor, validated by Microsoft (they test them to ensure they are compatible with BC and do not crash or generate errors) but Microsoft does not validate that the app functions as advertised.
We publish a number of apps - most of which are sold to our customers directly (we are a VAR that has a fledgling ISV arrangement) and to be honest I don't believe Microsoft can help in any substantial way.
There are a few Procure to Pay apps in AppSource, and an even larger number of cloud apps that are generic (not necessarily published to AppSource). If neither your partner nor the app publisher is providing responses, as far as I am aware you won't be able to get the source code from a NON CERTIFIED app.
Unless Certified the vendor DOES NOT share their source code with Microsoft.
It's not a good choice, but your best bet might be to find a new partner with experience in the area where you want support, interview a few of their customers to see if they understand your industry, and frankly switch partners and apps. Its not a good answer but I suspect that's where you are.
-Rob
PS: Reaching out to Microsoft people via LinkedIn is maybe a good approach. MS Owns LinkedIn and their staff will accept your connection. They will read your LinkedIn texts and might be able to help you navigate the bureaucracy.
