If your data is in UTC and you add the timezone offset, then it will work exactly like that.
1) I think there's a few different Tableau Online instances, but the one I've used the most is in US Pacific Time. When choosing subscription settings we just use the local timezone though (GMT+10 or 11 for DST) and it works as expected.
2) If you're extracting to Tableau Online it should be an exact copy of your database at the time the extract was done. This means that you could have outdated data pretty quickly if you're updating the database all the time.
The only problem I've found with Tableau Online and timezones is that when you check when the extract was refreshed it will show in US PST time, not in your local timezone.