Hi @Blindf8th ,
There is no Office add-in forum in Q&A.
For the first question, in my opinion they are 2 different stores. In Microsoft Store, it provides the UWP apps and this kind of app is operated independently, it will run the instance in task manager. But the Office add-in store, provide the add-ins for Office products and it run on Office products.
For the second question, you can try to ask this question to Microsoft 365 forum:
https://techcommunity.microsoft.com/t5/microsoft-365/ct-p/microsoft365
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.