If your PLC network is properly encrypted with an individual key (either entered into tpPLC utility, or randomly generated via button press pairing), then your router won't be able to see your...

This option, while neatly labeled "interference reduction" simply disables MIMO. It "may" reduce the data transfer rate by half. Of course, if your PLC doesn't have MIMO to begin with, there is...

You'll just have to pair them all, one by one, into the same PLC network so they can all talk to each other.

You want the router and your extra AP on separate channels so they don't ruin each other's signal.

That would be a normal use case, yes. Any PLC adapter can be used directly on the router, even those with an integrated WLAN access point.

... and very possibly, given TP-Link's track record, the older version will be abandoned very soon in terms of firmware updates and bug fixes.