Shepard the Leper wrote...
1. Immortal beings that simply "are" (according to Sovereign) who need organics (humans only in this case) doesn't make sense at all.
2. Why do immortals need reproduction?
3. WTF?
4. Sovereign says something different. Maybe they're both lying, who knows. Arrival also happens between ME2 and ME3. It isn't a real part of ME2.
5. Depending on Shep's background, your Shep might be the only survivor of a slave raid. Colonies dissapearing isn't something new and it's definitely not related to the Reapers. In fact, it would be an incredibly stupid thing to do, considering the Reaper's efforts to erase all traces of their existence. Colonies vanishing is a good way to draw attention. You don't want that 
6. Who made them "mindless"? And why? Slaves are always used to do the dirty work. Why do the Reapers need them?
7. The Collectors are Reaper builders now? They've done a poor job doing nothing for 50k years before starting to build one. Sovereign's failure was expected by the Reapers?
8. I've no idea where you got that idea.
9. Wiping out all life every now and then isn't a goal. That's sadistic, but that's all we know. Everything you've come up with is imaginary.
1. We don't know "how" reapers are constructed. We know they apparently reap organics' "essence" (whatever that is) in order to build another one. My best guess would place the first reaper as a post-singularity machine that "uploaded" an entire civilization into it, and is now in search for "friends". Why that is is still unknown.
2. This is a good question for ME3. Mind you, "immortals" is a matter of perspective. Nothing in the universe is "immortal", but by comparison with organics, this is a good characterization of a reaper.
3. I won't comment on the metaphysical shenanigan (WTF?? is a good reply), but again the human "goo" seems to include what is the essence of humanity. Don't read it mystically. Read it as (analogously) a caveman-like attempt to understand 21st century technology.
4. Soverreign says nothing about the human reaper. The human reaper is to be constructed with earth's humans, who will be reaped when the reapers get there. This is told when you enter the collector's ship and they claim that the size of it is so big that they assuredly are targetting "Earth". The only way to do so is with the reapers themselves.
5. This has been answered. When you reach Horizon, this induction is made by all your teammates, and it is a confirmation of what TIM guessed to be the case.
6. You answered your own question. Reapers are too "big" for these operations. They need "mindless drones", and organics that have been indoctrinated or messed up with are good "drones". This is also a continuation of the Cthuluh mythos, where something similar happened in that mythology.
7. Apparently they were waiting for the right organics to prove themselves worthy of attention and "reaperization". Apparently this "cull" happens every 50k years. You don't crop bananas every week. You wait for them to be mature.
8. It's what we are trying to tell you. Reaper's motivations were completely unclear in ME1. ME2 shows why they destroy civilizations every 50k years: to "post-singularize them" and make them into a new reaper. Further clarifications may be shown in ME3.
9. Apparently many intelligent people imagined exactly the same damned thing when they played the game. Quite extraordinary, isn't it? Perhaps we are witnessing a huge mental telepathic revolution! Or... alternatively, it's what the game tells anyone with a ****ing brain turned on when they play it. Let's guess where the Ockham's razor will "reap".