Not sure about the plane from the original post; it looks pretty hefty. And lateral weight is so close to the center of mass that it's unlikely to have much effect. What's more of a concern is having a bunch of weight far from the plane's center of mass, where the weight tries to lever the plane end over end, increasing the risk of a stall. I heard about a crash where the pilot's seat adjuster didn't lock, so when he took off his seat slid all the way back (just a few inches) but that was enough to cause a crash. Could theoretically happen to any size plane, but matters more with little light planes like those in general aviation.

I have no idea if "Spirit Airlines weight distribution issue" actually happened, but it's funny so I'll share: https://youtu.be/YvfYK0EEhK4

Magnetic variation in my area is +20° (west) off true north. So if I want to follow longitude line true north I need to fly such that the compass reads 20° NEN. And don't forget to account for the hunks of metal inside the airplane, which can affect the compass differently depending on your heading.