@NJRonbo 

The  backhaul is the connectivity between a pair of Deco.  Connecting Deco via Ethernet is indeed known as wired backhaul.  The previous models of Deco, when connected via Ethernet, are exclusively using Ethernet as backhaul.  The BE models are apparently combining Ethernet and wifi.

Each Deco unit is capable of acting as either a main unit or as a satellite unit.

Is placing a 3rd node on the bottom floor/basement a good idea?  A unit at a low floor will communicate with another mesh unit higher up if the wifi communication is good enough, which depends on the way your house is build.

You mention "Two of those nodes will be connected to Ethernet, from the same router".  This is allowed only if you run the Deco mesh in AP mode.  When the Deco mesh runs in router mode, one Ethernet port of the main Deco must be dedicated to the connection to Internet, no satellite can be accessible via that port.

Some good info shared here...

If you don't mind me asking how are your Eeros set up?  Does your ISP require you to use their router?  It may help us help you if you share what ISP?

1.  The BE95 has an extra 6 GHz-2, which is the same bandwidth as 6 GHz-1, some 11000 Megabits per second (and change).  Up until very recently there was a 1.0.21 firmware available that opened 6 GHz-2 to your use from dedicated back-haul, which means devices can connect to it vs just communication between/among DECO units.  Since you will have mixed wired and wireless back-haul I personally suggest the BE95 vs BE85.  That extra 6 GHz-2 can handle the wireless back-haul that you appear to need as well as wired back-haul, where the two units are pretty much identical.

2.  I split a 2 pack with a friend.  They are pretty much independent/exact duplicate units that can mesh together.  Maybe there's a different label at the bottom, the designated main router has a QR code for easier setup but otherwise hardware they are the same, and work in all functions the same.

3.  Yes units below or above can communicate with each other through WiFi.  I don't know if there's a difference between the flooring say main floor and 2nd floor vs main floor and basement, but I'm not worried.  You may buy the 3 pack first, and try out one down there in the basement, see if you are happy with the performance.  If not, simply move it to the 3rd location and not buy the 4th.  If yes you are happy buy the 4th and add the 4th where you wanted that one.

I bought a single unit, as a part of a pair that my friend and I split.  I don't have experience with more than one DECO so I have to refer you to the rest regarding their mesh performance...

By the way the automated forum censor is being really strict.  

  @jzchen 

Thank you, both, for your help.  

My ISP is Verizon and I am not required to use their router as I don't have their TV service.  I use them solely for 1GB Internet

I have 4 Eeero units and 1 beacon (an Eero extender).  Two of the five Eeros are connected with ethernet from the same router, at opposite ends of the house (see next paragraph). The other 3 units operate wirelessly as a mesh. I am running in BRIDGE MODE.  

 I buried cable in my yard and went from one end of the house to the other since the split-level setup made it difficult to go through the attic.

Since these are 2nd Gen Eeros, they only operate on WiFi 5.  I own WiFi 6 devices and I think the new iPhone is WiFi 7.  WiFi 5 speed in most locations is 350-400mbps.  I am hoping to see double those speeds with the BE95 mesh system installed. 

Good to hear there is no dedicated "main" or "slave" unit and that they are interchangeable, making it easy to buy add-ons.

I will probably go for the BE95.  The three-pack is selling for $1195 right now on Amazon, but I suspect it might drop further for Black Friday.  And, certainly, I'll try the 3 pack first and see if I need any more to cover dead spots.

Thank you again!