Thick mid-back, one pull at a time.
Keep your chest up and torso still, don't rock back and forth for momentum.
Pulling the handle to your stomach drives the elbows behind you, loading the lats and mid back from a stretched reach into a hard shoulder-blade squeeze, with the biceps assisting the pull. The seated setup with braced feet gives you a stable base, so the quality of every rep comes down to keeping the torso quiet and letting the back do the moving. Horizontal pulling like this is what balances all your pressing and builds the thickness a wide grip never quite delivers.
Both, they cover different planes. Pulldowns pull from overhead and build width, rows pull toward your stomach and build mid-back thickness. If you had to pick one for overall back balance, most coaches would keep the row.
Barely. A few degrees of natural movement is fine, but if your torso is doing a rowing-boat impression the weight is moving because of your bodyweight, not your back. Sit tall, and let only your arms and shoulder blades travel.
Tucked elbows brushing your ribs bias the lats, flared elbows shift work to the upper back and rear delts. Neither is wrong, they are two different exercises hiding in one machine, so pick the target and keep it consistent through the set.
