1) only for the whole mod, via XML\GlobalDefines.xml
2 + 3) Short version: No.
For 4 I'd not even have an idea how to possibly do it.
PHP:
<Define>
<DefineName>RIVER_ATTACK_MODIFIER</DefineName>
<iDefineIntVal>-25</iDefineIntVal>
</Define>
<Define>
<DefineName>AMPHIB_ATTACK_MODIFIER</DefineName>
<iDefineIntVal>-50</iDefineIntVal>
</Define>
2 + 3) Short version: No.
For 4 I'd not even have an idea how to possibly do it.