Nice ride and great photos!  It's such a different world out there then here on the east coast. 

I'm surprised that there is not much talk about Specialized tires.  Both the Renegade and Fast Track are awesome fast rolling tires that wear very well.  I use them on all my mountain bikes.  They work fine with Stans sealant also. 

I have an Garmin Oregon 450.  It was $200 from REI after sale/rebate.  Seems easy to use and takes AA's.  The screen is easy to read and it's been beaten around and works fine.  It's the only GPS of this type that I have ever used, so I can't really compare.

Overnighter with my 10 year old son on the C&O canal.  We kept it simple, easy and fun.  First time bikepacking for him.  It got down in the low 30's at night but he stayed bundled up and warm and never complained once.

This is why I'm confused. My tent is pretty light, and I'm honestly not gonna get a ton of weight savings with a bivy. I've never used a bivy, though, other than in the Marines. and I'm wondering how they are to sleep in the rain. I think that will probably keep me in my tent.

I've never done a race like the TD, but just the bikepacking that I have done, I would think a bivy would be better for an actual race.  Most of the bivys I or my mates have used weigh almost 2 lbs less then your tent and are much easier to deal with at the end of 12+ hours on the saddle.  I use a very lightweight tent for my bikepacking adventures that takes less then 6 minutes to set up, but the people that I have camped with that bivy spend less time getting set up.  In a race like the TD I can only imagine after days and weeks of pushing yourself that the little differences (like 2lbs saved weight with a bivy and less time to setup) start to become much bigger.

But don't let me discourage you from doing/trying anything.  From all the reading and following on this forum, it seems there are many ways to skin the TD cat.

The other thing to remember with regards to racing is that some areas are quite congested and it's not fair to the kids, horseback riders, or any one using the canal that some person goes blasting by them trying to set a record.  The C&O is a different animal compared to some of the trails that are "Ultra Raced" because of the amount of people that use it for recreation.

I faced the same decision for you and I decided to go with the Harness for the simple reason of allowing me to go on bikepacking trips in the cold and use my Mountain Hardware Lamina 0 bag that doesn't pack down tight enough for the sweet roll.  I think if I was racing I would use the Sweet Roll, but for general bikepacking the Harness and a dry bag is more useful.

I road the length of the trail last year in a ride from Reston, VA to Kent, OH.

The trail is soooooooooooooo flat that it numbs the mind.

Were you able to ride all towpath/bike path to Kent Oh, or did you have to do some road riding?  I have family that lives south of Kent that I would like to ride to, I'm in Frederick MD which is right on the C&O. 

I use the Rime's.  I started with flat pedals and when I bought my clipless I got the Rime's, so I can't compare them to the others you mention.  However I do deal with a lot of rocks and I've never had any issues slipping on rocks like my friends in their harder soled clipless shoes do.  The Vibram rubber that is on them is soft and grips really well, so I would imagine wear will be faster then the hard plastic cleats of other shoes, but after a full season on mine they still look fine.  Hope that helps some.