There is NO such thing as dual channel RAM and single channel RAM. There is ONLY, RAM.
ANY consumer RAM that is compatible with each other will run in dual channel if installed in a motherboard that supports it.
Buying in a matched set though, yes, that is a very good idea as it's guaranteed to work together. I wouldn't do it when you already have 8GB though.
It's probably worth at least trying to find another 8GB module that is the exact same part number as your current module, even though that is no guarantee they will play nice together. If you get a second module like you already have, and it does play nice with the other module, then it WILL run in dual channel in 99.99999% of cases.
If you want to get BETTER memory, in a 2x 8GB kit, then THAT would also make sense.
Not enough to bother with. Performance nor cost. I don't think that you can use higher than 2400 on g4560 so a set or single 3000+ stick would not help.
Maybe on a Z board but any budget motherboard H B etcetera will run at stock speed 2133 skylake, 2400 kabylake. Doubt the OP has a Z board in a budget build pc.
I do like the g4560 and 1050TI with 8GB ram, made for a nice budget system and I made one into a HTPC when g4560 launched and as a placeholder for six cores before we found out intel forced the 300 series motherboards. It was a half price I3-6100 essentially.
Who said anything about faster memory? I said higher quality sticks. If you're going to buy an entirely new set of sticks anyhow, getting higher quality sticks with lower latency and tighter timings, as well as higher quality ICs, isn't ever a bad idea.
As for speed, certainly they're not going to be able to do anything there. Not even sure why you raised that point.