Again, you're not answering the critical question. What mistake did the reloader make, if they were using published data, that would result in a cartridge being loaded "well over a cartridge's max pressure"? WAY TOO MUCH POWDER!!! In which case, the chrono won't do $hit because they are probably too busy collecting their fingers off the ground.
Chrono's measure velocity, period. Are they a useful reloading tool? Sure, but they don't provide a user with ANY pressure information. None. Nada. Zilch. Zero....
If you use a chronograph while working up a load, they allow you to see problems coming before they become a problem. If MV is getting higher than expected earlier than expected (given the reloading data you're working with), something is going on that might need attention. If it's more erratic than expected, something is going on that might need attention. A chronograph will tell you if MV is doing anything weird or unexpected, at which point you can stop and have a think. Before there are serious consequences.
It's not just about powder charge. Just about any other factor can get mixed up while reloading. Many of them have potentially serious consequences. I'm sure people make errors while reloading all the time, and usually there are no noticeable consequences because they were minor mistakes, and guns and cartridge cases are usually overengineered to handle it. Those people would never even know they were lucky. Chrono data might help them see it coming.
As I said above, MV data from a chronograph is technically a proxy measure for chamber pressure. But that's splitting hairs for most people.
If the exact same load is fired from a 2" revolver, 6" auto, and a 10" test fixture, will the cartridge pressure change? What about the velocities, did they change? What correlation do I make?
I'm going to stick by my statement. Use published data, exercise safe reloading practices, and always determine your own OAL so you don't have to worry about pressure. It's too late if you're hoping a chrono reading will save you from a catastrophic error.
Not sure of your point: Of course the pressures will be different. Every barrel's chamber, throat and rifling is different, and those have nothing to do with barrel length. You don't compare MV data across different barrel lengths, you compare MV data for your barrel length (which is listed in good manuals). But really you're interested in MV data specific to your particular barrel, and how it varies with your components and loading techniques.
Absolutely, nothing can prevent you from double charging (but as above, using bulky powders helps). Though I guess Trailboss can, if you're lucky enough to own some.
Absolutely, a chrono will show you if there is something weird going on with your load as you are doing the workup. Simply not being aware of it doesn't mean that there is no problem.