I think you are missing the distinction of seeing someone use a weave and then copying it and reading a residue of a weave that you didn't see made. Channelers can see other channelers (of the same kind, saidar/saidin) weaves when they are made, but to read a residue (maybe even to see a residue) after the fact is a completely different task. It is stated in the books that reading residues is a Talent.

Reading residues is a Talent and not everyone can do it. I think the strength required to make a weave has an effect on the length the residue lasts, but I think it can be a few moments to hours.

Yes, males can see Saidin weaves and most of the male battles have been fought at a distance instead of up close like the Nynaeve/Moggy fight.

Yes, if one can read the residue they can mak a gateway to the same place. It's the weave that dictates where the gate opens which is why if someone just opens a gateway without any thought in mind to where they are going it can open anywhere, even the bottom of the ocean. They didn't think of the bottom of the ocean, but their random weaving created a Gateway to a random place anyway.

If you are referring to the scene in Far Madding when Rand is trying to kill the Asha'man and Fain shows up, the Asha'man could not channel because they were in Far Madding, which is protected against the OP by a Ter'angrel.