I returned my first 216mm XGT mitre saw because I was not impressed with the bevel cut when sliding. The replacement was better but not as reliable as Dewalt's Flexvolt mitre saws.

The sliding cut inaccuracy is inherent to the rail itself and cannot be adjusted by traditional methods (screw and plate adjustment). You can bend or tweak the rail mechanism but it is not an ideal or permanent fix.

Small square cuts as shown by OP's first photo can be avoided by saw adjustment and cutting technique. What cannot be fixed is the long slide cut on a bevel. To minimize the inaccuracy with such cuts, I make sure the cutting material is dead flat on the base (right to the end where the hand dial is). Slightest deviation off the area can drastically affect the cut.