first of all, i'd also want to say that is a really well written and hugely informative post. thanks for putting all the effort into this!
a bit unrelated to this, but since you were talking about firing angles a bit i'd be very curious if you have any info how exactly these are calculated or chosen in-game? i tried to do a bit of research into this a while back here, but pretty much hit a brick wall.
Glad you found it informative and it's not just me venting to the void.
I don't know how the firing angles are calculated exactly, but I know that when a projectile has is_artillery set to "True" - which basically means that it travels in an arc, not in a straight line - the value in projectile_ext/speed seemingly doesn't affect projectile's velocity at all.
Now, bear in mind that everything below this line is my guesswork.
Before any calculation is done, all projectiles have the same velocity (value unknown to me). The game checks every angle (between 0 and 89, I think) whether the projectile will land at the target. Depending on firing_angle_type, it will either check from highest to lowest angle (high_angle) or from lowest to highest angle (low_angle and lowest_non_collide_angle, where the latter also checks if there are any shot blockers or terrain in the way). When it finds a suitable angle - search immediately ends, that's the angle it will use, the projectile is fired. However, in case there are no angles that land right on target, the game adds an increment to the initial velocity of the projectile dictated by the projectile_ext/artillery/speed_increment. It then once again checks all angles for one that will land on target. If there are none - repeat the whole process.
There's also speed_distance_multipler variable, which according to description is "Additions to the initial velocity based on distance to the target ms per metre ie inital speed modifier x distance to target". Almost every projectile in the game has this set between 0.1 and 0.3, but Sturmtiger and AVRE have them at -5. I haven't tested this thoroughly yet, so I'm not sure whether it actually affects anything.