I don't specifically know but I'll try to help. Which steps where you able perform?
Could you check under Apple Watch>>Setting>>General>>About>>Version? What is the listed version?
Maybe you have more stuff on your watch than I do. That would explain your memory footprint of 3.6GB. Maybe that is right where it is suppose to be, especially if you succeeded with the update and have version 7.x.
As far as Mail not syncing with the Apple Watch, check out Manage mail on Apple Watch - Apple Support. Perhaps "unsyncing" and "resyncing" the your accounts would help.
Also consider 1 800 SOS APPL. These people work on Apple products all day and would be far more knowledgeable than me. They have a front line of people to help with the obvious stuff, but if they can't help they have a far more capable staff of senior support people.