I solo in a 17'6" tandem, and am quite happy with it. I don't know if I would try heeling the thing fully loaded from a sitting position, but overall I find the handling issues to be minimal. Plus it's nice to have lots of room to put your stuff.
Of course it depends a lot on the canoe design, but I've only needed to worry about fine-tuning the trim when the canoe is empty: the bow can get blown around if you're the only weight in the canoe. But then I also have the option of kneeling further amidships if I need to move the canoe's centre of gravity forward. Of course trim is usually only a problem with a head or crosswind; with a tailwind you want the weight to be further back.
The only time I've found manoeuvreability to be a problem is when it's windy and I'm on a narrow twisty stream. You are heading around a corner with the canoe lined up perfectly, then a big gust of wind comes up and throws the bow out of line before you have a chance to react. At least twisty stream banks are usually soft.
You're not going to be as fast as either a solo canoe or a tandem, but the daily mileage hit compared to a tandem depends a lot on who your normal paddling partner is. For me, the difference is quite small (better not let my wife see this!), and depending on your fitness level, 12-15 miles per day should not be too difficult. You might be able to do 20 miles if you push it or have a bit of a breeze helping, and not too much portaging. (btw I use a regular wooden single blade bent-shaft paddle).
I usually triple portage, at least on longer trips. Rather than try to load up too much and risk injury on an unexpected slippery section, I prefer to take a light load on the first trip, scout out the portage if I'm unfamiliar with it, take some pictures, and clear out any low-hanging branches. Then take the canoe on the next trip, and the heavy pack on the last trip. I like portaging!

I've never tried reversing the yoke, but maybe I put in in backwards to begin with. I only have a single thwart (the yoke), so I like to put my head in the "bigger" space rather than the "smaller" one. Maybe the decision depends on the canoe design, maybe I just have a big head !
I've never tried lowering the seat either. I prefer to paddle sitting with my feet tucked under the seat part of the time, so I need enough room to fit those size 12's. With a lowered seat, you won't have that option, so you might end up in a situation where your legs are stretched out in front of you for the whole day, and your only recourse is to pull your knees up to the point where they interfere with your paddling stroke.
Remember too, that if you move the seat and then you want to go on a tandem trip, you will either have to move it back or your bow paddler will be stuck with your new seat height.
So I would recommend you buy the tandem and take it out solo without modifications. Then after a couple of trips, if you need to, you can think about moving stuff.