Just an odd question, born I'm sure of ignorance, but why do solo paddlers of open canoes not use the sort of two-ended paddles kayakers do? I'm sure there's a good reason for it, but to the outsider it looks an odd omission.
Lots of reasons, some historical.
To use a double-bladed paddle the lower hand goes through a low arc - the high side of an open canoe would prevent this.
Drips from the paddle blade are unavoidable with a double-bladed paddle. Over a days paddling you can get really soaked.
A single-bladed paddle stows very easily in the end of the boat.
Control with a single-bladed paddle can be better (with a lot of practise when you've learnt the strokes).
Multiple people can paddle a canoe with single bladed paddles. This would be hard with doubles.
Kneeling - this was asked about, above. Kneeling makes you more stable and allows for stronger strokes. However you can paddle perfectly well sat upright in a seat (even with a lower back support on some). It helps if there is something to brace your feet against.