Sorry to hear this is the way you feel. This is unfortunately not a simple case of adding primary keys to each table, and would be an extremely large undertaking on the software. Therefore is not something you are likely to see any time soon. However, I would like to address the below
I would suggest on this contacting a developers to work on this on your behalf. The documentation does indeed show the creation of members in the members section of the members section of the rest API here
https://invisioncommunity.com/developers/rest-api
A POST request would be sent to the following endpoint - /core/members
In addition, this returns the ID of the member that has been created.
You can pull a full list of members on the REST API if you need to pull those separately with a GET request to the following endpoint
/core/members
However you do mention the following after that part
Getting all IDs and the use of OAuth for creation of members are 2 different things here. In terms of OAuth and your other site being the client, you can create OAuth clients within System->Site Features->API for your purposes, which follow the OAuth 2.0 standard. All scopes are shown in that area that can be used, and again the ID of course would be provided to the client.
There isn't anything here at this point that you have said that isn't already available with our APIs. I would encourage you to get someone who is familiar with these, or if its just things you are unsure of, please post in our developers forum to ask those questions.