Copper and lead have different lubrisity (sp?) and the bearing surface of the bullets add to the way they resist the pressure. How deep each is seated, distance from the lands of the barrel, alloy used, all add up also. Weight is just one factor in how much pressure a load developes.
The pressure will vent in the path of least resistance. If that is toward the shooter then yes.
The burn speed of the powder is simply a measure of how fast that powder will burn in relation to another in a certain lab setting. The lab uses a "caloric bomb" to deduce this and the result has realy little value to a handloader. The burn rate will change in various cartridges, and isn`t a definate figure. The speed a powder burns in a 45acp may be faster then in a 9mm. If you look at a few charts the labs find different rates for the same powders in some cases.
The pressure also tries to keep riseing until the powder has all burnt. As long as there is still powder, gas is still being produced. Different powder will not only burn at different rates but some powders will produce higher total amounts of gas for the same charge weight. they don`t react in any kind of linier measure. The movement of the bullet down the barrel adds volume to the area under pressure and helps keep pressure under control. Any glich or change in bullet movement will cause pressure to rise as it is now not being vented as the gas volume increases.
Try picking up a Vihta vuori (sp?) powder manual. They have a very good detailed section on powders and their burning. The A-Square manual if you can find one is another very good manual for the explainations of causes and effects of component change and powders. Midway USA had both the last time I was at their web site. Don`t worry about the data just read the text and get a good understanding of what is accuring and how to use your tools to produce safe, accurate ammo.
Its only kinda rocket science.......