According to documentation, I can connect to my customer jira instance with an add-on. Is there another way to register my oauth2 addon with Atlassian so that we don’t have to involve each customer admin? One global oauth id and shared secret will work for us. This is crucial during sales cycle. Customer is not going to install addon for us when they are not sure.
Currently add-on needs to be installed which requires Jira admin involvement. If I understand you correctly you are asking about a way of installing user only integration using OAuth2 3-legged authentication. For now this is not possible however Jira has plans of doing that - https://trello.com/c/XfYS3iKU