What was Algernon's gameplan then? Answer: nothing. He was just as caught off guard by the whole situation when she announced her pregnancy. And he's a damn doctor who should have seen this coming. Then he spouted some fantasy about taking her away to some other country as if that would solve everything.
He got involved with a high society white woman from a rich family who was engaged to be married. Was it smart, did he think it through before he slept with her? Doubtful. And based on that by your definition he didn't take things serious enough either. And even if they moved to Canada or Europe which of the two would be giving up more? As a black male he would still be at the bottom of the totem pole regardless. Cornelia on the other hand would lose all family ties, all inheritance and societal standing and all the everyday privileges that normally came with being a white person of that era. She would now be all but shunned by the dominant white society because of whom she decided to make a child with. It would not be an equal sacrifice. Of course she would have gotten rid of her creepy father-in-law so maybe it would have evened out.
You can claim that she proved to be a coward or too weak when a decision had to be made. But played him? No. Not if it wasn't her intent to bail out from the beginning. And don't forget it was Algernon who told her before the breakup that their affair should have never happened. We'll just have to agree to disagree on this one.