a) I'm guessing we're talking about milliseconds, right? if that's the case, I'm fine with that.
b) I believe the module I have doesn't have inverted logic. As I mentioned before, for some reason those two relays stay open with either value. I've tried loading the sketch with the HIGH value and with the LOW value. In the first case, all 3 relays are on. In the second, only the relay in pin D7 is off. The other two, in pins D5 and D6 are on. I think it has something to do with the board pins more than the module itself.
c) when you are talking about external and internal in the board, what exactly do you mean? When I talk about external resistor I mean to add an actual resistor to my circuit, connected to the proper pins. Are you saying that as long as I don't use D0 as an input, I really don't need to solder any additional resistor?