There are two points:
1) Yes, we are assuming no one does 51% attack, but this assumption is based on economics and game theory (e.g. that BTC becomes worthless and hurts miners so it's not rational). Though it doesn't mean there can't be a hostile actor who's dedicated to bring Bitcoin down, and it's an inherent flaw.
-snip-
Exactly. However, recently we've learned that economics and game theory aren't accountable as there are hostile people (i.e. transaction spam attacks).
Where can we get more background on the pros and cons of each option?
-snip-
Well if you read the thread and you would know a few. Basically Gavin's proposal is risky mainly because of China and their bandwidth outside of the country. If we increase the blocks too much then it will cause even more centralization as miners outside of China will be generating more orphans. I think that a 2 or 4 MB compromise wouldn't cause such a issue.
There are also cons to sidechains. Basically that's completely unexplored territory, something that has never been done before. Sidechains are going to add a lot of unnecessary complexity for their benefits. It's hard to teach people already how to use Bitcoin, not to mention how harder it is going to be to explain sidechains and two way peg. Another factor is that we don't know when it is going to be fully operational. Even if we use sidechains or the LN right now, eventually 1 MB blocks will not be enough. I certainly would not want to be having the same discussion in 10-15 years.
Tl;dr: There can't be an agreement because nobody is willing to compromise. There are a ton of topics on here and reddit for you to read about the debate.
