2003 Ford Focus cutting out when idles

I'll let someone else handle your other question. For the ignition coils, I don't know of any mechanic who actually measures the resistance other than to verify one has an open coil. First of all, they go by symptoms. Simply due to the way the coils are wound, it is very unlikely the two ends will short together. The only other thing that can happen is a wire corrodes or breaks off causing an open circuit. That would read "infinite" on your ohm meter. You don't have that condition either, so electrically the coil is good.

In electronics, unless something is specified otherwise, the industry standard is plus or minus ten percent. Assuming your other coils are within specs, the one in question just needs to be within plus or minus ten percent of the good ones. Even brand new coils will all read a little different.

Most of the time when a coil fails it still reads the correct resistances. It's the insulation that breaks down and lets the high voltage cause a spark internally. Once it occurs inside the coil, it won't occur at the spark plug. Carbon tracks are left behind where arcing occurs, and that carbon is a conductor, so once it occurs, it will tend to continue and be real easy to find.

If the insulation breaks down just around some of the loops of wire, that part of the coil can short leaving just the remainder to generate the high voltage. Once the maximum voltage that can be achieved is lower than what is needed for the spark to jump the spark plug's gap, a misfire occurs. That seems to be the cause of all the trouble Ford has been having with their coil-on-plug ignition coils. Here again, the secondary coil is going to measure the correct resistance. Ohm meters only have 6 or 9 volt batteries so they won't cause arcing in the coil. During operation the secondary will develop up to 20,000 volts or more, as needed. That's when the arcing will occur.

I suspect you're using an auto-ranging ohm meter. I have over a dozen digital meters that I used in tv / vcr repair, and I never used those. When your entire day is spent taking voltage and resistance readings, you don't have time to waste waiting for the meter to pick the right scale. I know what to expect for a reading so I set my meters to the proper scale, but more importantly, the few times I've used other people's auto-ranging meters, I made too many mistakes by not paying attention to which scale it selected. It's hard enough to concentrate on the circuit and my test results. It looks like that happened to you too. The secondaries will have thousands of ohms of resistance. I'm pretty sure you overlooked the "k" or that the meter selected the "x 1000" scale. That is not the issue in looking for your misfire, and that happens to everyone. Auto-ranging meters work perfectly fine. Just remember to double-check your results when you come up with a reading that appears to show a defect. This is especially important when reading a sensor voltage where 3.0 volts and 0.3 volts are both acceptable values but mean wildly different things.

I forgot to mention, the best way to identify the ignition coil as the cause of a misfire is to swap two of them like you did, then see if the misfire code sets for the different cylinder. You can do that with injectors too, but it's mainly GM that has trouble with mismatched injectors. Since your misfire code didn't come back yet, you may still have a weak coil, but you moved it to a spark plug that has a slightly smaller gap and doesn't need as high of voltage. An ignition coil has a maximum voltage it can develop, but once that voltage builds up high enough for the spark to jump the gap, the voltage will not increase any further. If a coil, for example, is capable of developing 17,000 volts, but the spark plug requires 15,000 volts to create a spark, that is all the higher the voltage will get. As the gap wears wider, the spark will still occur up to that 17,000 volts. When 17,001 volts or more is needed, the misfire occurs.

I don't use those either. I don't think they'll work with a coil-on-plug ignition coil. Your best bet is swapping coils and injectors to see if the misfire moves to a different cylinder. Next, if your scanner has a record feature, you can record a few seconds of sensor data when the problem occurs, then play it back slowly later to see what changed. In this case you might look at the front oxygen sensor to see if it went excessively lean. That would be the result of a spark-related misfire. The unburned oxygen would be detected by the oxygen sensor as a lean condition. You can also look at the short-term fuel trim numbers. If they go high positive during the misfire, the computer is adding fuel because it thinks the mixture is too lean, again, due to that unburned oxygen in the exhaust.

Don't overlook the fact that it may just need new spark plugs. It's at that mileage where they could start to cause trouble.