How do I determine if pathway down inside wall is clear to connect wire to exterior outlet lower than the interior outlet. Also the interior box k.o.'s are hard to remove since it appears wood surrounds box. Then how to drill hole through the wood with box in place?
You will have to fish wire through... so you need a path. Professionals utilize a variety of specialty tools and special skills to do what you describe.
I would probably remove a KO and probe down the wall with a stiff wire to see if there was an obstruction. if there was, I would tear out the existing interior box, cut open the wall, use a long flexible drill bit to punch a hole, then patch it up.
You can spend a whole bunch of time fretting and trying to find an "easy way", or you can just do it and prepare yourself for the work involved patching it up.
When a box is mounted to a stud, and you want to go to the other side of that stud, you have to knockout one of the top or bottom knockouts (the furthest one from said stud, if you're running EMT or you won't be able to get it in there), and install a 90, which means as Jimmy suggests, you have to remove more drywall then you think would be necessary.
Big hole, little hole; same amount of work to patch it.
Drywall is the enemy!