Why can’t I block until I am attacked

I had no trouble doing all these immediately (beginning of chapter 3, I believe I did it) and in fact you can even max them all out in that chapter - unlike previous games, you can access pretty much the entire city except the very small Arsenal district from the very beginning.

What you might be missing, though, is that you can only have one mission at a time open. There doesn't appear to be any way to influence which den's mission will come up, but when you finish one, if you have a level 10 Assassin, the next one should crop up. This also goes for the level 14 missions as well - you have to take them one at a time. (Those are extra obvious because the Den Leader shows as on a mission in the assassin's interface even though they're actually still there.)

They'll wait for you to get there. I'm not sure if there are any consequences if you go forever without even visiting that den, but it doesn't seem like there will be...I can confirm that they'll wait at least several hours.

If you fail a den defense, the only consequence is that the den reverts to Templar control, and you have to kill the captain all over again - it's like you never had it in the first place.