The fence should not move and you should not have to tighten the lock nut down with excessive force. You should be able to just finger tighten the fence down and it should stay. I would suspect there is something wrong with the fence if it is moving on you that easily. I haven't used that brand of biscuit jointer, but the DeWalt and Porter Cable models have fences that lock well. Take it back to the shop where you bought it and describe the problem, maybe they can help.

I haven't bought a biscuit jointer yet (I used to use my father's) but depending on the forces exerted on the wood you should be able to edge join the pieces if you have enough bar clamps.