First image: For a highpoly it doesn't really matter. The square around the hole is unecessary but it's not exactly 'wrong'. There's also no need to connect those parts so completely with the rest of the mesh. Look at how n-gons behave when you've got subdivision (turbosmooth) applied. That should give you an idea of when…
Yeah that's better. What you're doing with sub-d is preparing for the subdivided mesh while managing a lower amount of geometry. Take for example the bent U metal bar that connects the body of the gun to the stock. That's currently highpoly. If you realise the bend needs to be longer or something like that, how will you…
I would like to see your .obj, please upload! On the right is the edited low poly, so basically I could have saved my base low poly before adding the supporting edge rings and go with this, instead of re-doing it by working the high poly to low poly? I think I'm still missing parts of the process, the typical workflow you…