1. is it possible to look through all the matching embark sites after using the site finder? after i've let it run it's course it just points me to one. but when i watch it run there are usually many spots where all conditions match.
Personally I would recommend not using the site finder, its too buggy now, or so I hear. It also can't find special features for you, so you are better off looking for things yourself. I am assuming you are using v31.
3 is it possible to get waterfalls in the open? the only ones i've found are where two rivers/streams meet. and the elevation of the higher region always overlaps the area where the rivers meet so that the waterfall is in a steep valley.
If I'm reading this right, this sounds like an erosion problem, after 1050 years, the waterfalls move back, making canyons and stuff, try aborting the history process around 100 years or so.
4. is there a benefit in above ground farming? when i look at the wiki all the plants i can have above ground seem to have a counterpart in the underground.
First off, you don't need to muddy ground for above ground farming like you do for underground farming, so you can start it off right away, right after you gather your plants of course... second, your dwarves enjoy a wide variety of booze, so using strawberries and prickle berries is beneficial. I usually start off with 2 3x3 plots for growing just those. Helps make ends meet if I have any irrigation mishaps...
5. are there any conditions for forgotten beast to show up besides RNG. i had one apear in the first cavern who nearly wiped out a fortress in the second year. i didn't even have a metal production up and running and my militia didn't get far with their wooden training axes. the human caravan guards managed to kill it though.
Conditions like population size? I don't think so. However, if you don't breach caverns, then you don't have to worry about forgotten beasts, because they can't get into your fortress!
I honestly have no idea for 2....