Firstly, why do all my buddies suddenly betray me?
They are mercenaries. Meaning that they will betray anybody for the right amount of money.
The facts and lifestyle in Africa show that most of the people living there are trapped, really unselfish or up for quick cash (I of course mean the problematic part of Africa and not the safe regions like South Africa).
Like the saying goes: TIA, bru!
"This is Africa", - meaning go with the flow/don't expect anything to run on schedule/enjoy life. Slang phrase. Appears in Blood Diamond (film) as well. There is also the film So This Is Africa, as well as the hit song "T.I.A." by Somali-born Canadian hip-hop artist and activist K'naan.
Secondly, how come all my buddies from Act 1 have come back from the dead?
They are actually not dead but MIA. I agree with you that this part was extremely weird. The plot problems, referring to the final mission, has been mentioned in most of the Far Cry 2 reviews.
Thirdly, why have they come back from the dead just to screw me over?
The writers probably wanted you to feel the TIA effect. So you would understand how your best buds can betray you for the right rewards. This actually is valuable knowledge in real life.
If you get the point, what the writers of the game wanted to make with the final mission, then you can see that really there are no alternatives. They couldn't make some random people betray you, only the ones who you "trusted".
If you really wish to understand what Far Cry 2 is about then you probably want to read the novels.