How to add OneDrive as an attachment location in iPad Gmail app?
I'm doing this via iPhone but the same findings should hold.

Question 1: I believe no is the answer, unfortunately. The closest you could get to having this button is also having Google Drive, sending from OneDrive to GoogleDrive from inside of OneDrive or a third party multi-cloud app and then using the Google Drive attachment button which exists within the Gmail app. The 'export to Gmail' option you've already found within OneDrive seems like the much better solution to me.

Question 2: I believe the answer to this one is that it's not possible to save the drafts. If you want to work on the email later and hence don't want to send it immediately, the best work-around is to send it to yourself and then edit it and send to the final recipient from your inbox.

Sadly I can't offer a better solution but hopefully this is of some help or at least offers some closure to your query.

Best wishes,

JB_199