I'm not going to respond point by point, but just make some broad observations that think you are mistaken about:
1. There is a distinction between "the GOP", and individuals within said party. Individuals act on principles. The party is just a collection of those people. It does not, itself, hold any principles at all. It also can't act in a unilateral or monolithic manner. Cause, it's made up of a group of individuals. I really think you're not grasping this. You keep lurching back and forth between talking about individuals within the party and what they should do, are doing, or could do, and "the GOP" as though it should magically just do what that small sample of individuals are talking about. It just doesn't work that way though.
2. Politicians often proclaim the worst case scenario when attempting to influence voters. And voters themselves poll differently when in a primary versus general. They do this because if they prefer candidate A over candidate B, they'll insist to everyone within earshot that candidate B would be a disaster, would fail in the general, will cause others to lose their races, etc. All of this is an effort to influence other voters in the primary. It's why you can't take the "if the general election were held today between Republican A and Democrat B" polls seriously until *after* both parties have chosen their actual nominees. Because if you're a Democrat and you want other democrats to vote for Sanders in the primary, and you participate in a poll running Clinton head to head against any member of the GOP, you're going to poll that the GOP candidate will win. Because you want eroded belief in Clinton's chance to win, in the hopes that other Democrats will vote for Sanders instead. The same thing happens on the GOP side. And guess what? The same thing happens with pundits and politicians proclaiming doom and gloom if their chosen candidate isn't elected (or the one they don't like it). Once the primary selection process ends, that tends to change dramatically.
3. Trump's issue with many on the Right isn't that he's too conservative, but that he's not conservative enough. This is what you *really* don't seem to get. More to the point, is that he's not really aligned politically in what we'd traditionally label as "right vs left". As such, attempts to apply normal election map rules aren't going to work. I've mentioned this a couple times, but you've failed to acknowledge it. Trump will certainly alienate some groups, but many of those groups already have low support for GOP candidates. You speak of "sacrificing principles", but the alternative path forward for those in the GOP would be to change their position on certain issues (like say immigration) to be more like the Left's. Um... That would be doing exactly what you claim. Standing firm on an issue that can be argued on a purely ideological and legal standpoint is far far better that what many GOP politicians have done in the past (standing firm on the more strawmanish religious right issues). That's the one that gets used as a boogieman by the left every election cycle. And that's one that is more or less eliminated in this race.
You talked about redrawing the election map, and needing to be competitive in traditional swing states. And you're correct. But I think you aren't getting that Trump has a ridiculous appeal among working and middle class blue collar workers. Folks who aren't happy with government regulations that have priced their labor out of the market, and have caused their employers to move elsewhere. Rightly or wrongly, they see Clinton as more of the same, and Trump as someone who at least appears to have something else to offer (I'm not even sure what that is, but then most of the voting public is dumb as a brick, so it's not really about having good policies, but rather making large numbers of people think you do). For every vote he may lose among the traditional identity/minority aligned voters, it's entirely possible he picks up 2 or 3 votes among that group. People who care more about their jobs, and being able to put food on their own table with their own two hands, and send their kids to college, and maybe even retire with something greater than the government subsidy that is social security. These are the people that Trump appeals to.
And yeah, I get that the left's response is to double down on the traditional identity based rhetoric. And that may just work. But it might just fail spectacularly. Again, that's rhetoric that they use every single election cycle anyway. So they get that, maybe a bit more of that. But they lose on a few other areas that I don't think they're thinking about that clearly. Trump is not your stereotypical conservative Republican. Not remotely. Treating him as one, who's just more vocal about certain things than others (and thus more likely to alienate), is a huge mistake IMO. I also would not be terribly concerned about a third party run from the right. Folks will talk about, and threaten it, but that's still a bargaining tactic. It's far more likely that Sanders chooses to run third party than anyone on the right. And I don't think either is very likely.