Contact details showing up for sources that have sync turned off
The Sync option only controls whether changes are synchronized to and from that device: that is, if you add or delete a contact on the device, this change is reflected on other devices, and vice-versa. Disabling sync just means the contacts that have already been synced to the device will get out of date when you change them.

To control which contacts are shown, you need to go into the People app, then choose Contacts to display from the overflow menu (the three-dots in the top-right corner). Then choose Customise from the Contacts to display dialog, and you can choose which groups of contacts from which accounts to show.

When you create a new contact, you can choose from the menu at the top which Google account to create it in (or to keep it on the phone only). It remembers your previous selection, so if you only want to use one account, you only need to set it once.