1 Length is only good insofar as it adds to meaning. Most length on LessWrong doesn't do that. For example, I can summarize your first point as:

Long comments make arguments clearer and make communication faster. Good communication is good, within certain limits, and I think most comments fall within those limits.

I don't think any important information is lost there. I disagree with your assessment of communication practices on LessWrong.

2 I don't think we should react to differences in tone the way that we do. The fact that our community has different norms depending on whether or not you use certain tones is problematic. We should try to minimize the impact that things like tone have. Substantive issues ought to be a priority and they ought to dominate to the point where things like tone barely matter at all.

3 Disclaimers discourage argumentative clash and take extra time to think of beforehand. Simply putting down a disclaimer allows you to marginalize issues that others might have with your post, it makes relevant criticism superficially appear less relevant. A better practice that we should be cultivated is to simply concede things after those things are pointed out.

4 The mindset of lines of retreat seems to stem from the idea that arguments are soldiers meant to defend your social status. Mental lines of retreat might be good but discursive ones are generally a way of avoiding responsibility.

5 Cross apply my above response to your argument about tone.

You say that they are good social skills. I agree, given the social norms of this site. But I think those social norms are detrimental to cultivating rationality efficiently and so I want to go about changing the social norms of this site.

But it's exactly things like leaving lines of retreat and using a polite tone that allows them to be less personally involved and not get caught up in things like having to "defeat" their "opponent".

I don't think so. At best, we've just changed the nature of the game.

EDIT: Upon reflection, this last point is basically the essence of my criticism. We've just changed the game to make it more superficially rational, but that is more resource intensive and it masks the underlying mindsets that are bad instead of actually changing them.