Hi Kevin,
Yes, you could hardwire your DAP-1522 to your powerline adapter in order to supply wireless signal to your downstairs area. You would need to set the DAP to AP mode. You do not need, or want, separate SSIDs. You are merely extending your existing network to downstairs via the DAP. No need to change anything else. Your 855 will continue to provide DHCP. You won't get the speed advantage of the DAP-1522 because you will be going through powerline. And don't forget, the more devices you add to a powerline network, the slower that network segment becomes. You may start impacting your HDTV with the additional traffic from the DAP-1522. If there is anyway you can get some ethernet cable downstairs from your DIR-855, that would be ideal. You could then connect your DAP-1522 to that and get some awesome throughput.
As for which device your laptops connect to, they should go with the stronger signal automatically which should be your DAP-1522.
Hope this helps.