Say I have backed up WhatsApp to Google Drive on some date X, then I uninstall it or format the device. On a later date Y, I reinstall WhatsApp and restore the Google Drive backup. Would the messages sent between dates X and Y be included in said backup?
How could somebody restore something that he doesn't even have? your Gdrive backup will restore the messages till date X.
If your WhatsApp was active between date X and Y then you would have already received the messages in your WhatsApp(whom you didn't back up on the Gdrive).
So on date Y if you reset your phone and restore the backup from date X then you will have messages up to date X only. and if you didn't use the WhatsApp by date X & Y and you just installed it again now then all the undelivered message between X & Y will be delivered right now.
Yes, the messages between date X and date Y are indeed preserved.
I have attempted this now, and it has successfully worked.
Do note that I was using the same phone and number, hence I do not know the success rate of this under a different situation.
Also, the old backup as it was does not contain the new data until and unless you update it from the settings.