Kozov: I’m not sure what your first sentence is arguing on. As for your second sentence, while there are mamrei razal that say such things, it is not for us to make the cheshbon of how quickly moshiach will come. I feel like this was addressed in a recent thread someone.
Anyway, since we don’t know when moshiach will come, it is impossible to know the extent that our tzedakah (in your example) hastens his coming. We have emunas chochamim that it will hasten his coming because razal said so. But how much so is impossible to determine. So our job is to learn Torah and do mitzvos and trust in Hashem. Moshiach will come when Hashem is good and ready for him to come.