1. Yes, it does, but we need to use a special item for this. 
2. Calculated items will read the existing data from the database and then create a new item with it.
3. No, aggregated items are calculated just like calculated items by Zabbix serve, but they do it for a group of servers. The zabbix-get tool can only retrieve information from Zabbix agents.
4. No, it's not a good idea. It's best to look for another solution to do this, as we've seen for our MySQL database.
5. No, it works on all items.
6. Yes, place the history at 0 days. Zabbix will create dependent items but won't keep the data from the master item.