I've been using this forum to get some Ford product knowlege. I've posted some things that have turned out to be embarassingly wrong. Here goes today's attempt.
1. On the highway (O2 sensors warmed up, closed loop, off idle) a vacuum leak (cracked manifold) would not cause an overall lean condition, the PCM would notice using the O2 sensors, add fuel appropriately, and it wouldn't cause any highway drivability problems due to the mixture.
2. Air in the manifold is air in the manifold. It doesn't care whether it comes in through the throttle body, a cracked vacuum line, or anywhere else. After the engine is fully warmed up, in closed loop, pull the brake booster vacuum line, far bigger than a crack in the manifold. Go for a drive. At idle, it may after 10 or 20 seconds get to the limit of the fuel trim and run some lean. On the highway the only way you can tell is you will need less throttle. This only applies after the engine is in closed loop.
3. Slight hedging on 2. Any air added to the intake manifold anywhere will reduce the vacuum, increase the manifold absolute pressure, everywhere in the manifold. Just like the air came in the throttle body. If the engine has intake runner manifold with long tubes specific to each cylinder (or pair), the drop won't be exactly even like it would be in a single plane manifold. But the difference would be far less than the pressure drop from the throttle body to the intake valve and that is very, very little drop.
4. The fuel trim in any misfire will go positive, that is a misfire in any cylinder will cause all cylinders on that bank to get more fuel. This isn't because of the vacuum leak, it is because unconsumed air from the misfire is being sensed by the O2 sensor. Again, this only applies in closed loop. If the misfire is from no spark, the aggregate fuel trim will only go up to 5-10%. If the misfire is from no fuel, the aggregate fuel trim will go up 25% or more and could cause carbon fouling of the other three plugs on that bank. Excuse, me, you have a misfire on both banks so all 6 other plugs.
5. A carbon fouled plug has LESS resistance to ground than the spark across the gap. If it didn't, fouling wouldn't hurt plug performance. In short, carbon builds up on the ceramic in the plug's well around the center electrode. This makes a low resistance path from the center electrode to the side (ground) of the plug. Spark takes this path instead of jumping the gap. This would put LESS strain on the coil instead of more. An open plug or too wide a gap could damage a coil but a carbon fouled (rich) plug wouldn't.
Someone please correct any mistakes I may be making here.