@Semp3rfi 

I'll link another thread from another ex-military guy redoing his church's WiFi in the next post.  In the mean time:

1. Approx. how many units would I need for full coverage, given that square footage?

Impossible to say, need to see floorplans, expected density of users, % of active users (not just connected).

2. Are the EAP245's Wired Ethernet Backhaul supported, Daisy chainable?  (I do have ethernet run throughout the enitre building).

You can daisy chain them, but what are you doing for power then?

3. Do they have captive portal, and mesh, without using the TP-Link Omada Hardware Controller?

You do need a controller, but it can be a free software controller running on a computer, a VM, a container...  I run mine on either a NAS or a Raspberry PI (I have an OC200 too)

4. Can I access them via a web browser or only a mobile device?

Yes.  You typically access them through your controller, either locally (ie LAN connected), or via the cloud (using TPlink's free cloud connect service).  You can use the App or a browser.

5. Are they backward compatible with some of our staffs outdated devices they use on the network?

The only troubles I've seen have been with some IOT devices, and most of those have been solveable.

  @Semp3rfi 

Sounds like you are on track.  I would agree with the 2 APs each for the Sanctuary and Hall areas, you may wish to consider 600 series HD suffixed models for these two rooms, ideally mounted on opposite sides of the room, or at least the opposite corners of the same wall if you can't.  Try to keep wall mounted APs at 8'-10' above floor.

The offices and other cinderblock enclosed rooms will be more challenging for coverage than capacity.  You are going to have to do a little experimentation here, but you could even probably get away with a mix of 200 series APs as the user density will be lower and tech requirement lower.  For better feature/future/function, I would probably do the 610 as ceilings and leverage the 615-walls for offices and additional coverage (latest firmware allows them to mesh now!).

My first guess (Plan A) would be to put an AP in the hallway between two doorways.  Fire it up and check signal strength and data performance in adjacent rooms, and then rooms next door (up and down hallway), you'll get a rough idea of whether having 3 APs on the ceiling or walls will cut it or not quite quickly (you would likely cut 2.4G radios on any additional APs installed in the hallway to avoid interference on ch 1/6/11).  I'd consider hardwiring a 615 in an office, this gets you a few things, not the least of which are 3 more wired ports via the integrated switch.  For rooms needing coverage but poorly serviced by hallway APs, a standalone 615 will mesh to a hallway 610 and provide much stronger signal in the room (as well as 3 wired ports).

Plan B would be to investigate putting APs in rooms with wires and seeing if you can cover all rooms with those.  Verify there's coverage in the hallway to avoid call drops or interruptions if people are doing VOLTE on their phones.