R6300V2 now considering Extenders

You've asked a lot of questions. Lets go over a couple things. 

 Extenders are a great idea. But in your case I wouldn't recommend them. There's a couple reasons why. 

• You're already starting from a low starting threshold of 15mbps. A single/dual band extender drops speeds 50% of what it receives. So ifs in a spots thats only picking up 10mbps from the original 15mbps, its only going to be putting out 5mbps. This happens because single/dual band extenders have to use the same chip to go router-----extender  and then extender----devices. And they can't do both at once. If you were starting at a speed poing of 100mbps, its a little different because you have some room to lose throughput and still have enough bandwidth to do what needs doing
• you're needing multiple extenders. whenever people are needing more than 1 extender, I usually recommend picking up a mesh system versus trying to piece together either a bunch of extenders that have different ssids or going with mesh extenders that might not be connecting right because there's more than 1. When you have an actual mesh system like orbi, you have a router controlling the system as well as you have a single ssid so you can roam throughout the home and not have to constantly switch networks. 

I'd recommend looking at the triband Orbi setups. Reason why is the tribands have a dedicated wireless backhaul between the routers/satellites that also supports wired backhaul. meaning it doesn't take that 50% throughput hit. When you're already starting at a low speed, ever mbps that you can preserve is going to help. You don't need a top of the line mesh system but you'd want one with the triband. You could go with a dual band but in that case I'd highly recommend hardwiring in the satellites to preserve those speeds. 

Amazon has been having a lot of newed RBK43 systems lately. That'd provide a router and 2x satellites for around the same prices as 2x extenders.