I will do my best to answer your questions.
but note, that I chose edge over etrex for one reason and one alone.
Power. I use powertaps/srms on most of my bikes and so that is why I have an edge. The etrex, despite having the hardware required, does not have the firmware required to read and record power.
As a cyclist, if you only need hr, speed and cadence, the etrex WILL do those things.
Now, onto the edge specific questions.
-Maps and Navigation: One of my main concern is the maps on the 800... Can you load the identical detailed topo maps onto the 800 that you would onto the eTrex series? Are these easy enough to be used for navigation? Including following a course, such as the ones available for download on CTR's website? I'm comfortable using a map and compass, and plan on always having a physical copy of the route, but on certain occasions(night time after a long day) it's nice to look on a screen and see a dot to verify your location, or simply follow a line on the screen for a while.
Edge 800 is the only way to go for mapping... The 500 will allow you to load a track like the CTR route, but will only display the squiggly line (I have both 500 and 800 because I do 24hr solo racing and switch units mid race - but there mapping doesn't matter, so I saved a few bucks on the 500).
As such, the 500 is not really a candidate, IMHO.
The 800 works great. I get all my maps from gpsfiledepot and in my opinion are better than most maps. They go down to the 10k topos and are highly usable. And you can use the garmin loading tools (mac and pc) to load only the maps you'll need to ctr.
-Storage: Will the 800 have the ability to store 6 or 7 days worth of riding information for upload later? I know it uses Micro SD cards for loading topo maps and stuff, but will it be able to retain all the ride information as well?
The edge 800 can store weeks, neigh, months of data. I run a large microSD card. I tend to load maps to the unit itself and save all my training files directly to the external card. (you can do that - tell it where to store it's files - on the garmin memory or the sd card). You will never run out of room. You can also load maps to the card as well. Memory is NOT an issue. Just get the max card size supported (cannot remember if it is 16gb or 32gb).
-Battery: What kind of charge will the AA's provide? When using the AA charger for the Edge 800, will one set of batteries bring it to full charge and then be able to run again for 14hrs? This then assumes that I'd need 8-10 batteries for a 5 day trip.
I use a 3AA charger. And if memory served... charging without running gave 100% charge. I do not know if you get a better charge *while* running it. I am getting ready to revisit that again. Because that was lithiums not alkalines. I might make a different charging solution using a customized mintyboost and some ideas I have for my setup to make it better... but ymmv, test it yourself. It *IS* workable thought. And there are places along the CTR to restock batteries. Lithium AAs are very light FWIW.
I actually went through this for my lighting setup and one thing I found is that carrying enough lithium AAs for the entire CTR was lighter than running a dyno hub + light.
The size and mounting of the Edge 800 are obvious perks. The price is a drawback, as it's nearly double some of the eTrex models. Hope you guys can answer some of these, Greg more specifically, as you're the first I've seen that's used the Edge for this...
The mount I thought was a weak point but turns out I like it more than I thought I would... It isn't as rigid as others... but I believe that is what prevents it from ejecting. In a crash, invariably, it rotates on the stem. But is still there.
I do carry an extra set of orings (which garmin includes) at the start of anything important. Cool thing is I just stretch them over the ergon bar ends and slide them towards the stem and they stay there for the race.
I hope that all helps.