But, to open a gate, requires a person on this side going to the site of the opening and performing some sort of magic ritual. They'd have to have sleeper cells in every city, some of whom are going to get themselves killed if they start performing rituals in the middle of Balmora. The Daedra's tactic appears to be to open gates in isolated areas and pour their armies through to fight conventionally... It makes more sense for the daedra to send small guerrilla groups through many different gates, which cause a bit of mayhem at isolated roads and settlements before coalescing into one large army for conventional warfare such as is implied to have taken place at Aldruhn.
I like what you're saying here. It makes (much needed) sense.
But this, I think, doesn't really hold:
[The Daedra's tactic appears to be]... on much rarer occasions, as in Kvatch and the imperial city, to perform lightning raids that devastate the city and then leave. Given the rarity of the city gate attacks, it makes more sense for the daedra to send small guerilla groups through many different gates...
"The rarity of the city gate attacks" is explained rather by the fact that the people learned how to close them than by the lack of means of intent. The game shows all the major cities of Cyrodiil under siege (however naive and easily broken), and there's no reason to doubt it was the same elsewhere.
Consider this: if it was possible to open "city attacking gates" in Kvatch and Imperial City,
why wouldn't the same be done in other places? If you have the manpower and resources to do this in (arguably) the largest cities, why bother with guerrilla warfare elsewhere? We do agree the goal is to destroy settlements, one way or another, don't we?
I
still like what you're saying. It could be made to work... if we more or less reject the game as a fanciful representation, which I have absolutely nothing against... Imagine that there were no gates near/inside the cities at all, only in wilderness, and the whole war was fought as conventionally as fighting monsters from another dimension gets.