Hmmm...comfort questions are difficult to answer. What's comfortable for me may not have anything to do with you and your comfort zone. I commute on a 2005 7700FX, and have a Specialized Roubaix Pro comfort bike, which is very similar to your Pilot 5.2. So I can compare your two choices at least a bit.
Your two bikes have some definite differences in terms of components, with the 5.2 specing out to the higher end than the 7.7FX. The 5.2 has an OCLV 120 carbon frame, with the 7.7 being mostly aluminum with carbon monostays. The 5.2 has a full compliment of carbon components and a full ultegra drivetrain. That puts the 5.2 fork and wheel components one to two quality/performance levels better than what you would get in the 7.7FX. In terms of the overall package, the 5.2 is definitely going to give you better performance over the road in terms of providing a smooth and plush ride. The 5.2 will be quite a bit lighter too, so it will be a bit more nimble, probably twitchy even with flat bars compared to the 7.7FX.
My Roubaix's all carbon ride defines plush, and I put some Bontrager Race X Lite wheels on it to give it an even better ride. It is definitely the faster of my two bikes, as it gives just enough of an aero fit to get you out of the wind fairly well. I'm 47 with some wonky back issues too, but I found that riding on a road bike is actually more comfortable for me than riding upright...go figure. (Most of that is due to finding it easier to make power from a road bike geometry than it is riding upright on a fitness hybrid. But that's just me.)
The 7700FX rides like a fast hybrid for sure. It is a great climber for a hybrid. But, you are definitely still riding at all times upright and in the wind. Your 7.7FX and the 5.2 should be pretty much identical in terms of drivetrain set up, feel and responsiveness. I do notice a trade off in the brakes on my 7700FX compared to what I am used to from a shimano based road bike.. They are a bit less responsive and precise for me, quite a bit more mushy. Some better pads will likely fix most of that though. On the road the 7700FX bike feels to me a lot more like riding a BMX than riding a hybrid. I am out of the saddle hammering all the time to get my power and quickness. You don't really have to do that on a road bike because of the differences in geometry and the ability to dig into your drivetrain power from the saddle...even on a plush bike.
The biggest trade off though on the 7700FX is that there is no way to really get out of the wind. You just have to accept it and deal with it. That coupled with the geometry differences make it really hard to have the same type of ride speeds as I would get on my plush Roubaix bike. I don't go that much slower but for me it's the difference in covering my commute averaging 16 miles an hour, versus the 19+ I would average on my plush bike riding in the drops. I have dropped into a couple of group rides on my 7700FX and could hang with the pack, but it took a lot more work than it would on my plush bike, a whole lot more work than it would on my racer road bike. If your group rides are comfort pace oriented, even with a bunch of people riding on road bikes, you would probably not even notice the trade offs. You would be working harder to make up for the higher wind resistance from sitting upright though.
On the other hand, the upright ride of the 7700FX gives you great visibility and great stability. I have tried...lord knows I have tried...to wreck out on my 7700 and I just have not been able to do it. It is an incredibly forgiving bike. Nothing about it is twitchy or iffy. You plop your butt down on it, hit the pedals and you definitely riding a bombproof bike well under your command. A carbon road bike...perhaps even with a flat bar...wouldn't give you as much of a confident ride. The 7.7FX frame is just that little bit more relaxed...look at a picture of it off the trek site next to one of the 5.2... to put you in a more upright position so that your center of gravity drop just a bit farther back over the saddle. So even over rough roads, it doesn't quite jar you the way that a road bike does. You are taking more of the road feel up through the frame and through the saddle. My plush bike deal pretty well with pot holes and pavement cracks, but a whole lot more of the impact is distributed up through your arms and shoulders.
The best advice that I can give you is to take your dealer up on their offer to put a flat bar on the 5.2 and try it out to see what differences it makes for you. That's the only way that you are going to know what is right for you. One thing that you can be sure of...getting a bike that's not comfortable is a mistake that you never want to make.
Hope this helps.