Hi, Tim:

Nice piece. You have shared much helpful information. Thanks.

1.  I think our local mesh was open for business in 2015.
Ditto.
I saw 2 open-to-the-public displays of the local group, 2015, 2016.
They were using LinkSys WRT54G devices.
I had one and a couple other 802.11g devices that accepted OpenWRT firmware,
but the were all indoor devices. I did my own testing indoors and outdoors
with tripods. I was not able to network. 
It appeared to me that the local group was not getting sufficient networking.
In 2017 the local group was, again, doing a public display.
This time they were using a Ubiquiti Nanostation.
Aha! An outdoor rated device. Now I believed a RF network could be built.
I was advised to obtain a Ubiquiti Bullet M2 and Omni antenna.
That combination did not work even as good as poorly.
Next, I obtained a Ubuquiti Loco M2.
I could 'hear' but could not establish LQ/NLQ with a nearby local high profile node ('hub').
I took my loco M2 mobile and grove all-around the high profile node site and
failed to get 'connected' (LQ/NLQ>0%).
With this mobile station I visited several other high profile sites (6) and
a few local hams homes.
I discovered that there were 32 nodes on channel '-2'.
The 6 high profile sites were all 'exposed'.
Myself and the other 25 nodes were nearly all 'hidden nodes'. 

2. "Our operating is on 2.4 GHz, Ch -2, with 5 MHz bandwidth."
Our operating was on 2.4 GHz, Ch -2, with 10 MHz bandwidth.
Today we have 1 user on 2.4 GHz on the 1 high profile node in Montgomery County,
In a neighboring county there are 6 nodes on 2.4 GHz, none of which are RF linked
with that county's high profile node.
All are tunneled. :-| 
Today's local map indicates 8 links of 2.6 to 13.5 miles with 0 hidden nodes
and 0 exposed nodes.
Of the 80+ nodes linked, perhaps 20 are RF, 30 are DtD, and 30 are tunneled.
Almost all of the tunneled are to distant for a RF link.

3. Chuck (Reply #4) Do tunnels and rf links have same effects on operation of the radios?  For 
example, if I have an rf node with four active rf links and four active tunnels, is that the same 
from a loading standpoint as eight active rf links?
Node CPU/resource 'loading', yes. RF links require more CPU loading than DtD/tunnel.

4. Chuck (Reply #5)  Hidden nodes.  In an AREDN network, is it necessary for all the nodes to be 
able to communicate with each other for the TDMA to work?  If an AREDN network has a star 
configuration with a central hub, will the packet transmitters eventually figure out when 
to transmit so as to not step on each other?  
I do not think AREDN uses TDMA.
I think an Access Point with Clients uses TDMA.
I think AREDN is 'ad hoc' and uses CSMA.
IOW, in an Access Point / Clients network, hidden nodes are mitigated.
In Ad Hoc networks, hidden nodes and exposed nodes are network destroyers.
A network with a 3 node 'star' hub must not have any nodes on the 
same or over lapping channels.
Amateur Radio has one 802.11bgn channel on 2.4 GHz not shared with Part 15 users.  

5. Ed (Reply #8) Regarding rf and dtd.
...
Will rf and DtD links 
between the same two nodes still mess up the OSLR tables if that is true? 
Although addressed to Ed, I will emphasize and
agree with Ed in https://www.arednmesh.org/comment/23549#comment-23549
There must not be RF and DtD linked nodes.
If co-located, they may be DtD. 
If not co-located, they may be RF.
Never both.

6. Jim (Reply #7) - I have attempted to attach a graphic showing the location of the hubs.  
...
Our generally flat land and trees are problematic for rf links.
Nice work with your graphic. :-)
Welcome to the masses of we in the USA forests...and valleys left by the ice age. :-|

7. The best rf connection we have from a house to a hub is a physical line-of-sight path from a short 
tower on a roof to one of the water tower hubs about a  mile away.  That hub's Current Neighbor 
list is one (tunnel, wan, active), one (rf, active) with SNR of 44/52  and Quality of 80%, and 
one (rf, blocked retries). A link to the other water tower hub failed during the day.  
Is 3-4 Current Neighbors the max that one node can be expected to support?  

Your group started like our local group in 2015.
6 exposed nodes on the same channel.
Our local network today has 30+ nodes with 0 exposed nodes and 0 hidden nodes.
Today, all our links are PtP. We have one 2.4 GHz high-profile omni hub for our 1
remaining user on that band.
We have one 2.4 GHz link, one 900 MHz PtP link, and the rest are 5 GHz PtP links.

73, Chuck