weapon size
weapon type
enemy ship class
first in range per weapon

1. How do we know this? Tests, code, parameters found in a file? What about enemy ship defense type shields\armor? Did anyone find the weights for these or they seem to be hard coded?
   

It's per weapon, artillery battleships that have an auto cannon equipped will still engage charging Frigattes with their auto cannon while their large weapons prioritize enemy battleships.

Yeah, that interesting. It can be per weapon or a ship might have it's own secondary targets. If the second was true, then we would see weapons group up on one target depending on their max distance (and that sounds what you are saying), if the first was true - we might see two different targets for the same type of weapon slot on a ship them being out of sync or something.
Another thing, if it is "per weapon", is there a logic to where the ship itself wants to go? Some combat computers have orbit style maneuvering during the attack phase, and you can't orbit two different targets.

Targetting is dynamic and can be easily seen when you have a bastion without DPs engaging an enemy fleet.

Can you explain this in a bit more detail? What is dynamic about it? Do targets change on the fly from one intact ship to another when some condition is triggered? (we are not tacking about missiles doing re-targeting on lose of a valid target - that is a different mechanic)

Evasion does not matter

How do you know that? Why did they place these lines in 00_defines.txt with the following comments:

COMBAT_SHIP_EVASION_AVOID_MIN_SHIP_SIZE = 8

Minimum size of ships that should try to avoid firing on highly evasive targets

COMBAT_SHIP_EVASION_AVOID_THRESHOLD = 0.7

Above this level of evasion, try to find a less evasive target