To much oil in a mower will make the pull cord jerky or very hard to pull. To much oil can be just as bad as not enough oil. Drain the oil out and replace with the correct amount and thst should solve the problem.
Your pull cord is jerking because your flywheel key is sheared, or your lawn mower blade is not installed correctly. To much oil should just make the lawn mower smoke a lot. Just remove the excess oil.
No. That is actually a problem with your pull start mechanism. If it is a 2 stroke engine, you need to take the top off and check to make sure that everything is aligned under the housing.