First of all: Why did the Observers choose to travel back to 2015 when the world fell apart in 2609? What was so special about that year that made them want to travel back, and since when did Observers become so violent? It seems strange to me that beings that are usually peaceful and non-interfering would suddenly change like this.
Secondly: Where is Olivia? It didn't appear that she was encased in amber with the others, so was she killed in 2026 (as we saw in "The Day We Died") or did she die earlier before the others were encased in amber? And is that why Etta has a bullet as a necklace; is it the one that killed her mother?
Thirdly: Did this happen in the other universe as well? I'm very curious as to how the other universe reacted to this sudden invasion, because if the bridge was still active then the Observers could cross unhindered. So was the bridge sealed off, or is the alternate universe under the same control that ours is in 2036?
Here's my thoughts on your questions above:
1. I agree with you totally here, up until this point, the Observers have essentially been like the Time Lords: pacifistic, with no desire to intervene. In fact, when August intervened to save that girl's life in I believe Season 2, they reamed him a new asshole for it. So yeah, it seems strange that beings that were this peaceful and with a seemingly strong moral compass in regards to non-interference, this invasion seems... Strange. Then again, desperate times breed desperate measures. So I don't really have an answer to this one, only that I agree totally with you that it just doesn't seem to fit the profile of the Observers we've seen before.
2. Dunham obviously died prior to Walter, Peter, Belly and Asteroid being encased in amber in 2015 (the significance of the bullet Etta wore around her neck. This would negate Olivia's previous 'death' at the hands of Walternate in 'TDWD', as that particular future has been changed by Peter's disappearance after going back through the machine. It's at least in keeping with what September told her that in all possible futures he had seen, Olivia had to die. So sorry, as cool as it might be to believe that 'our' Olivia escaped to the Red-Verse, I'm sure it didn't happen.
3. My guess is that the Observers, being able to travel freely between both 'verses and all times, have invaded the Red-Verse as well, and those folks have their own problems and probably could give a flying rat's ass about the Observers going Big Brother on us. This too, is something that begs more explanation, as does Belly's being around when it was stated earlier that he had died, and just what it is that Walter is going to do to save the day, now that his brain is unified once again. Though I would hazard a guess that whatever it is that he's going to do with Belly's hand, it might be connected to the other 'verse in some manner.
One thing is for certain: This ep is probably going to generate a lot of conversation between now and the finale.