Hi guys,
if I change a race's base attributes (STR, INT etc) and reactions, will this affect just my character when playing such race,
or will also modify all NPCs?
It will affect NPCs with autocalc stats checked, and new characters.
None other.
Example 1: I tweak Dark Elves to be stronger - will all Dark Elves NPCs be more dangerous in combat to reflect that?
Pretty much no. Any hostile NPC you meet as opponents have autocalc checked, and will have hit the attribute cap for strenght once you reach around level 15. Even before that the difference between strength 95 and 100 is not that noticeable.
Attribute modifiers are not the way to change the feeling or dangerousness of a race.
Example 2: I tweak Dark Elves to be -15 in reaction when interacting with Argonians (damn they were used as slaves since recently, that should be reflected in game) -
will Argonians now be more hostile just to my character, or to all dark elves NPCs as well?
If you tweak Dark Elves, then Dark Elves will react more hostile to Argonians. Not the other way around.
It does affect NPCs and the player character alike.