I have neither the time nor the interest in going line by line through somebody's post as is popular in forum debates, but this one line did catch my eye and I have to mention it.
Third, there's no reliable basis to assume that Dawnstar is virtually defenseless and even less so to speculate that it would remain so. The meager number of guards and population in general is a function of scaling to make the game playable and to scale the cities relative to one another.
To this, I have to say that I have every reason to expect that Dawnstar is mostly defenseless, as the existing example of Dawnstar is the only real representation of the town at all.
Make it bigger, and you've still got a sleepy town sitting on the water with no defensive structures of any kind, only... bigger.
You mentioned that Skyrim's navy would protect the north. That assumes that Skyrim has a navy worth mentioning. Do they? I've never heard of it, and while I don't doubt you'll say that you have, I don't really think that you have either.
We have heard a bit about the imperial navy, but since this hypothetical situation assumes that Skyrim won't be under the empire's protection, these forces won't apply in the equation.
Skyrim's guards complain again and again that the empire thinks that the nords are all lawless beasts. To this I say that a guy murders their king and half of the country sides with him. Just because people think unfavorable opinions about you are unfair doesn't mean those opinions are false.
I'd be very surprised if Skyrim's military ships were of the same calibur of the Thalmor's. For one thing, Summerset is an island nation that launched a successful military campaign that rocked the empire literally to its core. True, the dominion isn't entirely composed of island nations, but while they tell us that the bosmer are a part of the dominion as well, I am left with the impression that they're not the driving force behind it - based partially on the lack of bosmer Thalmor represented in Skyrim, and the lack of Bosmer representation in Dominion documents. Arrogant though the Altmer are, they're also really intelligent, at least according to the game (I'd happily argue that there are a lot of idiot altmer). Altmer ship designs are likely far ahead of the curve.
Nords are stubborn to the core - represented in the series many times. Nords do things because they want to, or because it's a sacred tradition. You can't swing a dead cat over your head in Skyrim without banging it against a sacred Nord tradition. As a people they tend to be full of bravado, courage, and pigheadedness, and short on patience, level-headedness, and problem solving logic skills. If you do everything by tradition, your technology and techniques will naturally be behind the times. This doesn't bode well for Skyrim's fighting vessels, assuming that there are any.
So back to Dawnstar's example, if we simply made assumptions about Dawnstar based off of what we want to be there and not what's actually represented (or at least hinted at) in game, then where do we draw the line? Sure, a Dawnstar with a fortified port and an active military force, defended with dragons and maybe an army of sharks with freakin' laser beams on their heads - that could truly be an effective deterrant to invasion.
I just see no indication that any of this exists, or should be included in the discussion.
