5.1 Full Audio Setup

Short answer: nope. If fed a stereo source, Sonos will "upmix" it to 5.1 and send simulated surround sound to the rear speakers and the low end to the sub. AFAIK, you cannot turn that off.
You could pair the 2 play 5s in a stereo pair as a separate room, bond the sub to either the playbar/base and then group the two rooms together, which will give you what you seek. However, the delay on the room containg the play 5s will certainly be noticeable to the point of being unusable to watch TV.

Even if the play 5s are in the same room with the router and a boost?

To be able to synchronize playback of music between groups, Sonos introduces a buffer of about 60ms. This makes sure that when playing music across multiple rooms, it will sound unified. This buffer also ensures that differing wireless conditions between the different room pose less of a problem for grouped rooms.
For the TV inputs on the Playbar, playbase and beam, such delay is not used, in order to preserve lip-sync. Bonded speakers in a 5.1 group communicate directly to each other through a separate 5 GHz connection, instead of through the existing network, to facilitate the bandwidth this requires. However, as soon as you group another room to the room playing the TV audio directly, the added group will sound delayed, no matter how good the connection quality - this is just the reality of how the system works.

If the play 5s were hard wired via Ethernet to the router would that make any difference regarding the delay?

No, the delay is a fixed amount, in software. There's no way to affect it, either shorter or longer.

Still waiting for "full channel stereo" option....