The Dragon Twins do not pop up if you haven't completed Dragon Rising. To be revealed as Dragonborn, you must kill Mirmulnir during the attack on Whiterun's western watch tower. Even all the dragons perched atop Word Walls outside will not appear until that happens.

This is true. There have been *no* dragons in my current character's game other than Alduin appearing at the intro execution. He's even been up on one of those mountains with the dragon icons, symbolizing that a dragon roosts there, only to find three frost trolls instead. He can obtain the words of power whenever he comes across a word wall, but can do nothing with them because none are unlocked. Those at the execution are more likely to believe that Ulrich, the Empire, and/or the Thalmor are the cause of the dragon appearing and would be extremely unlikely to view one's character as a Dragonborn at that point.

Yeah, It's only the hit squad I object to (and only when it occurs prior to your interference with the Thalmor activities). I have no objection to the other two types of Thalmor encounter -- they add nice flavor and are not out of place or inconsistent with other events in the game. but the hit squad is, especially if you read the note they are carrying.

I agree with MadCat about what it takes to be revealed as Dragonborn. Further, I don't really agree with your logic about the Thalmor sending a hit squad after your non-Dragonborn character just because you became powerful, although I do understand your point. But even assuming the Thalmor were out to eliminate you as a potential threat because of your exploits, the Thalmor hit squad should not come after you until at least level 20 or so. Level 8 is too soon to have realistically completed Dawnguard or become much of a threat. I still think the best trigger is completing the Thalmor embassy quest or rescuing the Graymane boy from the Thalmor prison.

But the bigger issue I see is the note they carry. The note the Thalmor hit squad carries says they have targeted you for execution because you have "actively disrupted our activities and caused great harm." So, the rational of a prophylactic assassination of someone who could be a potential threat but has not "actively disrupted" any Thalmor "activities" does not fit with the way it was implemented in game.

The targeted note I assumed was due to the Embassy debarkle I hadn't noticed they attack before that. I assume its simply due to lazyness or oversighth on Bethesda's that they couldn't add a basic conditional quest completed check to the spawn event.