My 2 cents ...

I personally don't know the Swift Keewaydin 16, but based on its layout, it seems the most easily adaptable to solo use.  If you sit in the bow seat and paddle the boat backwards, it looks like you are good to go.  All of the other canoes would need to have the "permanent" yoke removed in favor of a removable option or don't lend themselves to a "paddle backwards" option based on thwart location vis-a-vis the bow seat.  (I assume you are trying to avoid paddling from the normal stern position which would be somewhat problematic in a cross/quartering wind.)

dd

I'd say that the Wen Solo Plus is likely your best bet.  IT already comes with a third seat, and I've heard it said that it is better at solo than tandem - I've never paddled one

I have a Northstar, and have only solo'd it a couple of times kneeling at the rear thwart - it is my Quetico tandem - it was OK kneeling, but not all that comfortable.  I suppose you could replace the kneeling thwart with a seat, but I think it would be better placed a bit more towards center for sitting.   (I have 5 boats - that is the only tandem, so I'm prejudiced in favor of a solo rather than a tandem for soloing - that may color my opinion a bit.) 

There are a couple of other boats I can think of still available new that fit your bill of goods.  One is the Mad River Malecite- symetrical hull small tandem (often cited in the tandem/solo discussions) the new version in Kevlar light looks really pretty - similar in specs to the Northstar and Solo Plus - I think it used to be offered with a center seat option.

The Souris River Q 16 is another tandem offerd with a center seat options - i.e. you can get it set up as either  a tandem, or as the Q 16 solo version, or maybe with 3 seats. 

If you can find one on the used market, the old Dagger Reflection 16 was another boat touted for tandem/solo use and was offered wtih a factory center(3rd) seat.  Mad River makes the boat now, but its not the same boat.

If you are buying new, I'd see about special ordering a boat set up with a center seat and no bow or stern seats ( if you aren't going to use the boat tandem), or with a third seat option. 

If the thwarts are going to be a problem, you'll just have to reconfigure the boat for a center seat that works for you.  "Center" isn't accurate - most solo boats have the front edge of the seat at the balance point, so the seat is behind center.   For portaging, you would have to use a removeable portage yoke, or use the front of the seatframe in a Knu-Pack type of packframe rig, which is what I use on two of my solos.

My longtime "solo" canoe is the SR 16 tandem.
Removed the stern seat so I can slide packs farther forward to trim and reversed the yoke. Not as fast as true solo BUT I value stability more solo than speed for the way I trip.

MP is right about the width issue.
I'm a dedicated Double Paddler so I just use CANOE double paddle which for a tandem is a 9 ft one.

I still use the single paddle for slow trolling/backup paddle and have one that is long enough I can still paddle in the center of Bow seat solo.

You just have to make "adjustment" to accommodate your new ride.

The Wind IS a big issue solo and even more so in tandem being paddled solo.

I see pics of tandem paddlers with the packs standing up, which is fine maybe for 2 people, but you do that in a solo, your talking "sail" and whatever wind direction except maybe a tailwind, you're going to have even MORE control issues.

Even with the "learning' curve involved using a tandem to solo, for me, the tradeoff is stability is worth it.

On MORE than a few occasions, I've STOOD in my "solo", knee's braced against the yoke for balance, in shallow/muddy water that would have been hard to wade, and like a Gondolier used my double paddle to POLE my way thru.

I don't think you could do something like that in too many true solos

Another thing about a tandem vs true solo, you can ( in my experience) get thru shallower water because of the increased hull size/buoyancy.