Can Someone please help me if my understanding is correct with API Portal and OTK integration.
It sounds like you have a good grasp on this. For questions 1-3, your statements are all correct.
Once the integration takes place the OAuth manager is only used for token management. You will still see the clients tab but it will be inaccessible and no client list will be returned.
As for item 4, nothing additional is done with respect to pushing the API Key and Secret to the OTK DB.