Hi Brianw3185.
Welcome to THR!
As already mentioned, you need to get a manual. Lyman or Hogdon has a little more precise velocity data. That doesn't mean your results will be the same, but it will give you a better data set to compare patterns. Hornady manual is ok for thier bullets, but they give velocity in 100fps increments, so its a little harder to compare thier data. There are other manuals by bullet makers such as Speer, Nosler etc. You can also get information from the powder mfg of whatever powder you're using.
To answer some your questions which may or may not have already been answered:
1. Yes, Generally the same bullet in different weights is gonna have a lower max charge for the heavier bullet. For example:
Xyz 124gr fmj has a max charge listed at 5.5 gr of Ricky Rocket fuel and 1150fps , the Xyz 115gr fmj is likely gonna have a slightly higher max charge of say 5.6 gr and probably a slightly higher velocity of , say 1190fps. Pdq's 115gr fmj may have a little different construction and have a max charge of 5.1gr
So if you study enough data you can get a sense of how to extrapolate, but it takes diligence and caution.
There are also ballistic software apps that can take your powder, bullet, barrel length, etc, and give you theoretical computed load data.
2. Just about every bullet mfg can give you a recommended col for thier bullet, but that doesn't mean it will work in your gun. I have a G3c with a tight throat, so some bullets have to be loaded shorter than the recommend col in this gun. This means I have to be extra careful if I'm playing around the max end of the charge range because shortening the col can have a similar effect to increasing the powder charge. However, just like a higher powder charge, shortened col can cause pressure to spike suddenly at an unknown point, which is bad. Some powders like Hs-6 are pretty forgiving of this in 9mm, other powders like Titegroup are not.
3. Case volume is not much of a concern in 9mm unless you're a competitive shooter, but to answer your question, variation in case volume does have an affect on ignition and powder burn rate, and thus affects pressure curve. This is more of a concern with rifle cases, but it's worth reading more about it to understand.
4. Working up a load means starting at a known safe starting point, preferably published in multiple sources, and working up in small increments until you get the result you want. You start at a safe pt and work up in 0.1 gr increments and test in your gun. You are looking for flattened primers or other clues to make sure you aren't hitting excessive pressures. You do this until you find what you want , or find signs that you need to back off.
Velocity is a good indicator of excessive pressure, so a chronograph is usefull here. If you're seeing 1400fps from a load that the manual says should be 1200, that's a good indication something is amiss. If you make incremental powder increases, but suddenly you are not seeing a velocity increase, that's another sign you are in dangerous territory, or you have a powder that can't do any more in the barrel length you have.
5. I think you can probably get some data from Midwest, but I haven't tried personally. Someone else will probably have experience here and chime in .
- and get the basic books. 
