Well, you could argue that the gods they worship are mainly Cyrodiilic gods, even though each temple is specific too one god, they all still follow the Nine, and considering Talos (One of the Nine) was from Cyrodiil, and everyone knows about how Akatosh relates the the empire, so i can see Knights of Akatosh and Arkay joining, and it makes sense the other ones would join, mainly because of the Divine Crusader.
The Nine is a Cyrodiilic pantheon, however, you have to remember that there are political reasons not to join in. THe Nords, Redguards, and elves all have their own pantheons, and the Nords, Redguards, and Elves are all invading Cyrodiil. The knights in question are the militant arms of Temples. For them to specifically take such a political position, they risk being chased out of the provinces and completely crippling the effort to spread the Nine divines religion. The only ones who can risk joining are the ones who are based nearly entirely in Cyrodiil. Everyone else risks massive political backlashes. It doesn't make any sense for them to risk severe losses when they won't be adversely impacted by the loss of the Septim Empire.
For your second point, about how the KotN, when you say that defending the Empire isn't revelant, which isn't true, Sir Amiel states that it's the KotN's duty to protect the Nine, and the empire.
I can't find any evidence that a nearly 300 year old ghost said the KoTN's duty was to protect the Empire. (I'm still looking, but I don't believe he says that any where)
But Still, Address every part of the Argument why don't you? You still haven't addressed the fact that the Knights of the Nine includes members from the INVADING provinces, and that the first incarnation of the order's destruction was guaranteed by members going off to the War of the Red Diamond. These are both reasons for the Knights of the Nine to stay out of the war.
Also, the Knights were extremely famous the first time, and everyone knew the the stories of Pelinal, the KotN, and Umaril. When the quest is finished, you can hear the Knights talking about how they want to spread their influence, and make their mark in Tamriel, so everyone knows who they are, so its reasonable to say that they succeeded.
It's been nearly 300 years since the first Knights of the Nine, they were practically forgotten by the time of the Divine Crusader. Secondly, you base the conclusion that everyone knows who they are based off, what? That they want to spread their influence and make their mark? That's not evidence of anything except intention. Third, it has only been ten years and they started off with 9 PEOPLE and a small building in the middle of nowhere. They'd have to build new buildings if they wanted to house recruits.
Based on the lore, I see no reason for all the religious Knightly orders to unite.
I still think that:
The Knights of the Nine themselves would be lucky to have, at most, 100 men.
At most, I think the Empire can count on just over 300 knights coming to it's aid. (This figure includes the Knights of the Nine)