Can stent be used to open 100% blockage in an artery such as LAD during cardiac catheterization procedure. Cons and pros please. Thank.

Yes, if needed. Total blockage may occur suddenly - this is what causes a heart attack. Then the goal of the procedure is to get the blocked artery open asap (<90min) to save otherwise dying heart muscle. If the blockage is chronic, then it should be opened if the supplied muscle is alive and the patient's active lifestyle brings out symptoms. Stress scanning may be helpful to determine this.
Depends. What you are referring to, is opening a totally blocked lad long after it has closed off. Depends if there are collaterals ( natural bypass vessels of the heart) developed beyond the blockage, and also if you have a stress test showing that you have "ischemia" beyond the blockage (i.e. Living heart muscle that has lack of blood supply). Ischemia positive with inadequate collaterals = open the lad.