To answer your questions:
1) No. They are not cumulative. Best modifier is used.
2) See 1)
3) The Dominions 2 site list available from
Arryn's site or
Sunray's site gives information on terrain types, rarity and whether or not a site is unique. A similar DB for Dom3 is under construction, but is currently lacking a crucial bit of info regarding the terrains due to the vagueness of the mod manual on the site terrain masks.
Edi