Unfortunately, velocity can be a poor indicator of peak pressure and that's the kind that makes guns come from together, especially so if the powder is too fast for the load. Like, a very low velocity would come at a very dangerous pressure with a charge of Bullseye. Yeah, that's a pretty extreme example but I use it to illustrate the principle that we really can't be sure of the pressures by simply recording the velocity.
In my experience, increases in velocity (in rifles) tends to flatten as charges exceed normal levels; pressure quickly goes way up for little or no velocity gain. That's what makes attempts to gain more velocity by simply cramming in more powder so dangerous. BUT - if you have attained the published velocity BEFORE reaching the book max for a normal powder, that IS a good sign you have reached the full standard pressure, so stop there!
Best to stick with case observations and bolt lift to judge pressure, IMHO. But back off at least 5% if you see any of those signs, they are for sure indications of being far over max.
Again, velocity is the best indicator of pressure in the absense of any real pressure testing equipment. That is not to say that, for example, in a 30-06 shooting a 165 at 2850 that the pressure will be the same or safe regardless of the powder used. You must be able to interpret your pressure from the velocity based on the components you used. This information is obtained via published data. It's why the max load for each given powder is at a different velocity. Its why in cases such as the 30-06, that the slower the burn rate the higher the velocity at a given pressure the manufacturer has set as it's ceiling. But, still, when using a particular powder, your pressure is indicated by the velocity, relative to that particular powder and bullet weight. At a given velocity, pressure will increase as the burn rate of the powder gets faster, until at some point, that velocity is no longer being acheived at a safe pressure. Conversly, at a given pressure, slower burning powders will give increased velocity than faster ones. Still, the velocity is the primary indicator of pressure, in the absence of actual pressure testing equipment. If you're working up 30-06 loads with R-15 or R-22, your velocity is going to tell you when you've hit max, but that max safe velocity is going to be lower with R-15, but never the less, it will be the primary indicator, at least until you start going over safe pressures.
You are correct in stating pressure isn't linear. Also agree that if you hit max velocity prior to max powder charge, you've hit max. Conversly, if you haven't hit max velocity at a max listed powder charge then you haven't hit max. The max safe velocity will depend on the particular powder and bullet combination, but the velocity will tell the story.
As to your last paragraph...the consensus among many gun writers, and others in the industry, most of which have spent significant time in labs with pressure testing equipment and guns, is that the "classic signs" are not realiable. Many of them can "appear" and give false information. Cratered primers aren't always an indicator of pressure, but can indicate a large firing pin hole. Factory rifles often still have burrs in places and can leave shiny marks on brass. Different manufacturers of primers have differing hardness. Heck, many a factory round has flattened a primer. However, when the classic signs do actually appear, they are quite often significantly over safe pressures. Most say @ 70-72K psi in the rounds that have a SAAMI max of 60-65k psi like the .270 or 30-06. Meaning you can be shooting loads over safe pressures, but without them showing any classic signs, but any change, like a very hot day, or shooting the barrel hot, can result in catastrophic pressures. SAAMI max pressures are set with these deviations in mind so that you can load to SAAMI max, and the expected "normal deviations" will not put you in to thermonuclear pressure territory. Further, most data, is set a little below SAAMI, so there are 2 "cushions" built in to most data.
