Good for you for getting the bolt out. If you run into tricky problems like that in the future, an auto-machine shop has the specialty tools and expertise to fix that kind of problem in a jiffy. Given the risks you take on by inadvertently turning a minor, inexpensive problem into a major-expense problem, it is often better to just tow the car to the machine shop and have them do the repair job. The machine shop fee should be pretty reasonable for something like that, likely less than $100.
Note: If you still have an oil leak afterward, you may have damaged the valve cover, assuming that you overtightened the bolts b/c of the foot-pounds vs inch-pound mix-up. A junk-yard valve cover is probably the best bet in that case.