Does the strap favor either side of the body?

the two bottom straps are both anchored on the corners so you can swap which shoulder you want the bag on by swapping which strap you use for main and which one you use for retention

Will this fit a 15 inch laptop?

I have a MacBook Pro 15 inch, the one Apple released end of year 2017. Like one guy said, define fit. I can get my MacBook into the largest compartment yes, but it is pegged out length and width wise. That is, you can tell where the corners of the laptop are. If this bag was just that one inch larger it would be a perfect fit for a 15 inch laptop. It does fit, but its so snug that Im a bit uncomfortable with it that way. Im looking for another bag but will certainly keep this one as a bug-out or day bag. It is a great sling bag.

What is the Velcro loop on the back for

The Velcro loop on the back is there so if you store something in the lower sleeve opening, such as a tablet, or notebook, it is less likely to fall out.