i think this is a common “misconception”, that comes from other loot drops we know from other games. in most games gold items are “legendaries” which are often indeed stronger because of their rarity. i think a while back thomas said something that those yellow items are not like “legendaries”, more like unique items.. so i would say unique =/= legendary.
as for your other questions, i do not really know how enchantment rolls on those weapons work. someone else posted here: Possible bug with Golden Rewards and Enchantment Rerolls? - #3 by Schweinebacke
that enchantments are rolled in groups, so some groups of enchantment affixes have a higher probability than others. but i can’t say for certain how exactly the numbers are here.