a few Qs:
1. Golden age? I used to like this in earlier versions but in BTS somehow i don't see a benefit? Gold/science rises a bit, ok, but maybe it's better to use the GPs to build/discover/etc something or join a city?
I'm surprised to hear you say this. I find that in earlier versions Golden Ages were under powered, and in BtS they have been made so much better.
From memory I think all you used to get was
1. An extra

on every tile already producing 1

2. An extra

on every tile already producing 1
In BtS you also get
3. Free civic and religion changes
4. +100%

production
5. +100%

production
6. Extra wonder in BtS - the Mausoleum of Maussollos extends the length of golden ages by 50%.
These are not insignificant bonuses, especially the free civic changes when used efficiently. Plus, in BtS the cost of a golden age starts at only 1 great person, rather than 2.
Whether it is better than using a great person for a bulb/special building/etc is debatable and depends on the situation. But particularly just after a succesful war and you need to get newly acquired cities into shape, or late game particularly when building Spaceship parts, a golden age can be an excellent use of a great person.
2. Why can't i see all the rivals in the score/power/etc graph? I've met them all. Is it something to do with espionage or maybe a bug, cause in earlier games i could see them.
Now part of the espionage feature (push CTRL+E or click the icon for details). It comes from the passive ability "Can see demographics", which is fairly cheap in most cases, but does require you to have invested some espionage into your rivals.
3.
Which leader would you say it's the most treacherous? I found 2 religions but switched to Buddhism just to make Shaka happy(ier) but he just kept coming. Lucky for me i discovered a pattern. Kublai would strike (understandable as he was choked hard from my culture) from one side, and after i take a couple of his cities and garrison there to put down the revolt, Zulu would declare war and he's on the other border. Still, it's fun.
This post by DanF5771 deserves some sort of medal for explaining this sort of stuff.
Charlemagne at Pleased has a 100% chance to NOT PLAN to go to war with you according to the XML, and also can't be bribed into war with you at Pleased. Barring some random event, I think you are reasonably safe. But if he drops below Pleased, even for a single turn, you should be careful, as during that 1 turn he could start PLANNING (We have enough on our hands right now, aka, WHEOOHRN) and not necessarily declare until quite some turns, at which point you might have him at pleased or even friendly. Fickle fickle AI...
4. Which unit promotions give a better effect for attacking? A star (10% strength), a city raid or one of those like 25% vs mounted or gunpowder? I guess a star is like univalue and has best use for whatever unit, a city raid useless in the field and the gun/mounted well, i used them a lot but they don't seem very effective.
Mix your promotions up. Assign some units for stack defense with a wide variety of specific counter promotions, and other units for city attack if that's what your plan is.
This article by Arathorn from the war academy is a valuable read.
5. More a frustrating conclusion than a Q: Why does my 1st settler almost always appear on the far end of a continent and usually just a few clicks from ice?
5.1. Why when playing Terra map all civs appear on a single continent? And "America" is so freaking far away.
5.2. I have the feeling that every time the AI strikes with its strongest on my weakest and i do the opposite (not by choice of course).
5. Yeah... pretty random I think.
5.1. I think that is the whole point of the terra map type.
5.2. I'm not sure I understand the statement... Attacks your weakest city with a strong army? Sounds sensible to me. Attacks your weakest neighbour? Also sounds sensible. Sometimes you might do the opposite, attacking the strongest AI simply to remove the threat as soon as possible. This would require a bit more forward think than the AI is capable of. But if it is just more land for you to expand onto, the weakest neighbour is definitely best.
6. SAM inf. Don't you think they come a bit early in the game? By the time i get to planes almost every rival city, fort and stack have them.
Yeah, SAMs can be a pain if you're using planes. Personally I'm not a big fan of planes anyway, and as far as defending against land based threats SAMs are pretty poor, so usually I'm quite pleased to see them defending against my artillery and infantry.