If the threads aren't buggered up too bad, you might can run something through from the other side and straighten the threads enough to install the pedals properly. A pedal might do that if the treads are long enough to go all the way. If not you'll have a hard time finding something long enough that is left threaded.
However if you did not notice that things where right and left threaded or that you were cross-threading, then maybe you should go to a bike mechanic with this.
DIY is better learned when everything is correct. Otherwise, you'll probably get some misconceptions right from the start that will be hard to rid yourself of.
And still, we are a little confused about you saying you can swap left and right cranks. Are there any part numbers or brand and model stamped on the cranks? Again, how about putting a pic in the gallery and we'll look for it if you can't now directly post them in your replies here.