I guess the first thing I would ask is,when you tested for spark with a plug in the coil,did you ground the plug? A lot of todays electronics in vehicles can be damaged if not grounded when a plug wire etc is removed.Anyway if you didnt I would try it again while grounding plug,or buy a spark tester,is a very handy cheap tool.
If possible check the wiring to the coil for an open,if the previous test still showed no spark and the wiring to the coil is ok could be a driver in the ECM not firing for that cylinder.
If there is power/spark to that cylinder,I would say try a compression test on the bad cylinder.If compression test is ok,could possibly be a faulty injector.If noid lights are available they are a quick easy test to check wiring to injector.
Vacuum leaks?? I guess one thing I learned from working in the automotive field for over 10 years was to not overlook anything,no matter how trivial it may seem.
The flashing mil is an indication that damage to a component may occur unless problem is fixed,in your case the catalytic converter,if you still have a mostly stock exhaust.
I just want to say that by no means am I a Ford expert.I am not very familiar with their setup even though I have owned mine for about 3 years.Just wanted to say that so if I gave misinformation that you know I am not a Ford master mechanic and to avoid getting flamed for trying to help.I like to be able to try and diagnose my own problems with my own vehicles and finally got myself a Snap-on scanner a few months ago and purchased the updated cartridges with the latest software.Have been wanting to hook up to my car a check the system to see what info is available.
Anyway,good luck and I hope you get your mis-fire figured out...