Powerline adapters do not have IP addresses to begin with. Update the firmware, yours is ancient. After you've done that, pair the adapters again, either with a new, random passphrase via the button...

Try to access your router's DHCP log.

While the latest firmware for the 8030P has been a massive improvement on throughput and stability over previous ones, I do second your request for an update to qualcomm base MAC firmware 2.2. The...

The simplest way is to look at the DHCP server log in your router.

This looks like one of your neighbours started to use powerline equipment. Redo your own powerline network using a new passphrase.

Yes. See instructions for how to add more devices into an existing powerline setup - it can be done through the buttons on the devices.