According to what I have understood from your question, It seems like you want to purchase product only once.
What I wished to ask: is there a way to request information about consumed products like about owned products.
you can get response or it's information in any type(Managed Product, Unmanaged Product, Subscription) in the onActivityResult method() check my below link.
In-app billing-v3 error in activity result
but I would not prefer you to manage it customly as you told for one time purchase product(not consumable product). You should go with the Manage product.
Managed In-app Products:
Managed in-app products are items that have their ownership
information tracked and managed by Google Play. When a user purchases
a managed in-app item, Google Play stores the purchase information for
each item on a per-user basis. This enables you to later query Google
Play at any time to restore the state of the items a specific user has
purchased. This information is persistent on the Google Play servers
even if the user uninstalls the application or if they change devices.
for more information about product type
you can query any time you want and you will get the product information, and even you don't need to manage if user has already purchase this product or not.
Is there a way to restore cnsumed purchases or I should return payments to my users?
Better option is you should give the payments back to the user and for the next time check the whole app once using dummy product "android.test.purchased" and update your app.
Is there a wat to find out how many and which purchases were consumed?
You have to check in the Google wallet because all transaction should be handle by the it, check if it gives you product type or not. Using your google developer console credential you can signed into google wallet.
If returning payments is there a way to find out if user stil has the product or is it consumed with out changing the app (through google services)?
as per above answer you have to check in google wallet, according to my knowledge they are provide us all of information about product type with user detail.
Let me know if I have not properly understood your question.
Hope it will solve your problem.