The following is just speculation based on playing Plant Tycoon for a year or so, since I've never actually bothered to prove my theory. If anybody has tested this, feel free to jump in!
I've also noticed the appearance of flowerless plants while breeding the same plants over and over. My observation is that a batch of seeds produced by a "parent" plant will, in turn, produce plants that bear varying numbers of flowers. The "value" of any particular plant seems to be related to the number of flowers that it produces - plants with many flowers sell for more money than their more sparsely flowered siblings. Now, for my theory: I think the "flowerless" plants are the same as their siblings, but have produced zero flowers instead of the more normal number of flowers that you usually see.
As I said, I've never bothered to test the theory, since I usually wind up just selling off those flowerless plants and only pollinating plants with the highest value (and the greatest number of flowers).
For your second question - my experience (and my breeding notes) indicate that the commutative property holds true in Plant Tycoon, so A+B=C and B+A=C.
