Well you can make pretty much anything in 2D with Stencyl as far as i am concerned. Now making an Othello game is possible of course, the only factor can think right now that may be hard, is for you to get a suitable scale of the board.
The logic for an A.I could get complicated if you want a really really good A.I, but you can always keep things simple with only a few moves for the A.I to choose from.
I think Stencyl can help you do what you are searching for but it will require some effort.
As for how long it could take you, i should say, it depends on how complicated you want your game to be and how fast you can learn to use Stencyl.
I wish you good luck on your project!