Well... I have to say that at first read, the thought of someone trying to tackle an open water swim not knowing how to swim freestyle... scares me.
But... you went out there, and you can do it... without swimming freestyle. And... if all you want to do is FINISH... then it sounds like you can do that.
True, you might be the only one swimming back or side stroke... but if you don't care what others think... then they can screw off.
It sounds like you are motivated... and your mind set is that you can finish... and that is half the battle!
If you do it, let us know how it goes.