Does alarplasty narrow the nose? Does it also make nostrils symmetrical? Would the scar be noticeable?

Yes you are correct. Alarplasty can narrow the nose and make the nostrils more symetric. I like to place the incisions at the junction of the nostril and cheek to help camouflage the incissions. In most cases the scar can be easily covered with make up until the scar fades.

Alarplasty is often done in conjunction with Asian rhinoplasty to correct wide base of nose or asymmetric nostrils or it can performed as a stand alone procedure if no further modification to the nose tip or bridge is desired.  Scarring from this procedure is minimal when well executed.  There may be a noticable faint pink line on the skin for the first 4-6 months, which can easily be covered with makeup until normal skin color returns after 6 months. 

Alarplasty, or alar wedge resection will narrow the lower part of the nose quite nicely.  The scars are in the alar crease and typically heal imperceptibly.  The Alarplasty, when done well, is a great procedure. And can even be done under local anesthesia.  Attention to symmetry is a key part of the procedure and can even correct some pre-existing asymmetry.