Even side mirrors will not reveal blind spots, a head turn is the only way to check all angles in real life, and in LFS.
Yep, this is a good technique to master and can also force your opponent into an error or over-defensive action which can open the door for you to skip past. While one guy is losing the bulk of his momentum with threshold braking and restricting his options by taking a defensive line, the following car can get a faster and cleaner run out of the corner. Setting up your opponent for a pass is much better than crossing fingers and barging through. More racers should choose their time to pass strategically rather than go for it at the first opportunity.
When a car disappears from your mirrors, quickly check left and right to find their position. Combined with sound it is not too hard to know where they are relative to your car. It can take some practice to get used to the head turn but it works well enough I think. I used to go off line with the head turn but now I can do it without any errors, practice I guess. By touching the button very quickly I can get a half-glance left or right which is often enough to see your opponent and takes a fraction of a second to complete.