3 Questions regarding restoring the backup of whatsapp from Google Drive
so I understand there is no option to restore and save my WhatsApp history on the memory card. Could you confirm that, please?


Like any other apps, it is possible to save Whatsapp on the memory card. If whatsapp is installed on the internal storage, then its default location is the internal storage. If it is installed on the SD card, then it saves everything on the SD card.


  If I uninstall the WhatsApp application now, do I have the option to install it on the memory card and so restore my backup on the memory card where there is plenty of space?


Yes. If your phone allows for the installation of apps on the SD card, then there won't be any problem.


  If I uninstall now WhatsApp without choosing to restore the backup but also without selecting the opposite, will I have the option to do it the next time I install WhatsApp or is it now or never?


What do you mean by "If I uninstall now WhatsApp without choosing to restore the backup but also without selecting the opposite"? 

When you unistall Whatsapp, the backups are still present in Google Drive. If you then install the app again, it will ask you to restore them.
However, note that:


  Every time you create a Google Drive backup using the same Google account, the previous Google Drive backup will be overwritten. There's no way to restore an older Google Drive backup.