You seem to be very understanding and very patient, S&W can't ask for a better customer than that considering the issues you have experienced and delays in using your Shield.
Since you own other brands as well as seven Smiths no doubt you have experienced a few issues with some of the others that likely worked themselves out yet you have one 'problem child' that is yet to be right.
I don't know the logistics of issuing you a new gun, might be a bunch of red tape to go through on that? Perhaps you can find the right person at S&W to hear out your story as well as your loyalty and perhaps they can offer a 'loyalty voucher' that basically equates to most of the cost of new gun and they can keep your loyalty and allow you to purchase another Smith?

Other alternative since you really like the gun as most of us do that own Shields ( I own a Shield 45) is to take it to a legitimate gunsmith to evaluate. And if he/she can determine the cause for failure and fix it for an acceptable amount you could have that completed and then attempt to get a reimbursement from S&W or simply enjoy your gun and move on though I realize it would be difficult for S&W to earn your business again unfortunately.

There isn't a whole lot of complexity to gun like the Shield to a true gunsmith............I bet it is fixable or at least diagnosable Could it be as simple as faulty magazines?
Already mentioned but you might like to try the 45 LOL
Karl