1. Yes - The WRT1900AC is my DHCP server. DHCP is turned off on the EA6500.
2. The EA6500 is configured as a repeater bridge in the DD-WRT firmware. I'm no longer running stosk Linksys firmware on this router because the repeater and repeater-bridge options are not available with the stock firmware.
3. Yes - the SSIDs on both the WRT1900AC 2.4 GHz and 5.0 GHz bands are unique as are the SSIDs on the two bands of the EA6500 - I have 4 unique SSIDs on my network.
4. I have a Mac so I use iStumbler to verify that there is no channel overlap in my area.
5. Already doing this on the 2.4GHz band - set to 20 MHz on channel 9.