They messed something up during the oil change. I am not sure what without seeing it but I am a small engine mechanic and things don't just change when you have an oil change!I would have another
service look into it
You don't need to do all of that to test spark, just go to harbor freight and get a spark tester for under $5. You don't have to remove the spark plug that way. If I had to guess you may have bent one or both of the push rods
Probably the pulleys are worn out, I have seen this more often than you might think. The pulleys are worn too deeply so the repair shops put the next size smaller belt on and tell you it has to wear in.