Does having higher Reaction + Insight make the Light Weapons skill rise faster as it is used in the game? Or do they only affect the starting value of the Light Weapons skill?

Or is the skill increase for all skills tied to the Talent capability only? Ie, if you have Talent at 10, will all your skills increase at the fastest speed as you use them? 

If so, what's the point of the other capabilities because they would affect only the starting values of the skills and even if you max a capability out, the relevant skills can only start with a value of 10, a negligible amount...

(This is for ERAS 2 mod, but I guess it is relevant for all Maelstrom mods)

I don't see a difference.  I compared the function GetCharacterExpRate(ref _chref, string _skill), in file Program\characters\RPGUtilite.c from COAS and ERAS, and the logic within both are the same.

The SPECIAL_S, SPECIAL_P , SPECIAL_E, SPECIAL_C , SPECIAL_I, SPECIAL_A , SPECIAL_L correspond to the PIRATES scores, and are allocated as seen in Program\characters\characters.h file:

#define SPECIAL_S    "Strength"
#define SPECIAL_P    "Perception"
#define SPECIAL_E    "Endurance"
#define SPECIAL_C    "Charisma"
#define SPECIAL_I    "Intellect"
#define SPECIAL_A    "Agility"
#define SPECIAL_L    "Luck"

They correspond to the PIRATES descriptions, as seen in file RESOURCE\INI\texts\english\common.ini:

string = Strength,"Power"
string = Perception,"Insight"
string = Endurance,"Endurance"
string = Charisma,"Authority"
string = Intellect,"Talent"
string = Agility,"Reaction"

string = Luck,"Success"

There is a point in the code where the xp gained is multiplied by the ratio of skill divided by 10. It makes the xp gain 1x if the skill is 10. It makes it 1/2 x if the skill is 5 etc.