1. Initially, as it is in all retail technology, "smart guns" would be cost prohibitive for most people. Citizens then do NOT BUY "smart guns", they will buy DEPENDABLE mechanical guns.
2. The vast majority of law enforcement agencies, get the funding from the appropriate level of government - State federal county or city - who, in-turn, get their money from the taxpayers. Many LEAs now have MRAPs. If they can afford MRAPs, the cost of these magical crime-stoppers is irrelevant.
3. Requiring handguns to contain "smart gun" technology would be the death toll for all firearms companies who did not have an extremely lucrative government contract, and the fallout from that would be the end of availability of handguns (other than pre-existing, mechanical handguns) to the general public. Legislators and the public should give NO SERIOUS CONSIDERATION to this technology. An EMP protective holster would have to be a tightly-sealed Faraday cage, which would add bulk and weight to the LEOs belt, and impair rapid accessibility to the firearm. The mortality rate for LEOs would skyrocket.
4. I don't see where any constitutional conflict would come into the picture. "Smart guns" would still be "arms" to bear, but the cost would put them beyond the reach of most people. The constitution does not address prohibitive costs in any fashion. Following that train of thought out, even IF "smart guns" became the rule of law, the terribly shrunken market for that technology would cause the idea to implode without the aforementioned "extremely lucrative government contract". It would be a financial disaster for the arms manufacturers, which may be the actual intent of such a government mandate... UNLESS, they then approached the government, hat-in-hand, begging for a "bail out".
5. "Properly shielded" takes us back to the Faraday cage, already covered in 3, above. Your suggestion of a mechanical fall-back mode if the electronics fail is an excellent idea, but one that would add even more cost... but, WE pay for it, so what the hell?
Bottom line? I believe "smart guns" are anything BUT a smart bet for arms manufacturers, consumers, or government(s). They would be rejected by the civilian market for any number of reasons, primarily the realistic potential for electronic failure. It has been proven repeatedly throughout history that every "whistle and bell" we put on a product, the greater the failure rate. Guns "ain't broke", so don't waste any time and money trying to fix 'em. Guns do NOT commit criminal acts of their own volition - they are inanimate objects - the actions are those of the person holding the gun. That's my 2˘ worth. Pax...