First - Memory Clean, seemingly a useful tool, mostly because of the name, I suppose.
Turns out, it's not particularly needed, ever. Not a scam, per se. It just doesn't do anything useful that your system doesn't already provide.
You also asked when it is a good time to purchase more memory... Now would be a good time. You can upgrade to as much as 16GB. Even doubling to 8GB total would make your system work more smoothly, with a bit of apparent speed increase.
The "Other Volumes" is a listing that is ordinary, and normal. Not sure exactly why it is listed as "Other Volumes" but I have the same.
This thread is way too long... Did you ever swap out your hard drive for an SSD? 'twould be a wonderful thing, and also would delay your possible future laptop replacement even a little bit longer, as your MBPro would work like new.