So, for your data question. One of our technical writers released an article just yesterday about data quotas. Take a look and let me know if this is enough information for you, if not I will reach out to them for more specifics.
For your second question, you could potentially do an auto-save feature on an interval like some games do which would only lose a little of their level if they forget to save.
I also know that in JS you can detect the attempt on the user to close the browser or navigate away to prompt them to save. I have never attempted this in WiX/Velo though. My guess is you would need to add this in the scripts in the dashboard perhaps?