I always keep thinking about another research I've read about ([0] seems to be an article about it that I found just now, but I haven't searched hard for any rebuttals) that diverse communities experience less trust and less civic engagement. Basically, it makes sense that similar people want to stick together, and personally I see no reason why we shouldn't let them! (Conversely, those that prefer to live in diverse communities should also get the choice.) Another example is, plenty of Brexit voters knew that Brexit would be bad for economy, but supported it nevertheless because they expected other benefits to outweigh it, so I don't think arguments concerning the economy should overrule all other considerations.
Btw, I disagree about free speech - it seems to be pretty dangerous (career-wise, if not life-wise) these days to offend people (see Tim Hunt, Donglegate, Justine Sacco).
I guess my broader point was embedded in my original post - these questions have multiple answers, but not necessarily the right answer. People have different opinions and priorities, and they're not necessarily better or worse, just different. I think we should respect that and accommodate this kind of diversity ("opinion diversity") into our society, not try to exterminate it by shaming those that disagree with us and trying to change their minds. This is a great article that describes such a world: [1].
[0] http://archive.boston.com/news/globe/ideas/articles/2007/08/...
[1] http://slatestarcodex.com/2014/06/07/archipelago-and-atomic-...
On diversity, we have ample evidence that it makes us smarter[0] and more productive[1].
I think that placing offense and free speech into opposition is a false dichotomy. I think it's better to offend people than to not offend people, although I'm hard-pressed to think of what evidence I'd offer for that other than to ask: would you rather be offended, or not offended? I rather suspect that most people would choose not to be offended. So if we choose to value non-offense, various issues flow from that, although I haven't seen any impact on the exercise of free speech (as evidenced by our recent election).
On immigration, again, it turns out we have evidence for immigration being a net gain for the US economy[2]. There remains some question about the localized effects of those gains. Anecdotally, I suggest considering just how much of the Silicon Valley economy we discuss on this site revolves around companies started by immigrants or the children of immigrants. It's pretty staggering.
After that, we start to get into questions of cultural which seem more fraught with peril, and I believe the questions assume too much, so I'll stop. I'll say only that these questions you claim are forbidden seem to be things I encounter just about daily online, despite answers being freely available (not just assumed), so I'm not sure how effective the forbidding has been.
[0] https://www.scientificamerican.com/article/how-diversity-mak... [1] https://trailhead.salesforce.com/en/workplace_equality_diver... [2] http://www.epi.org/publication/immigration-facts/