Don't think a caliber swap would be possible from a shotgun to centerfire rifle or vice versa.
Apart from the obvious barrel diameter issues, the shotgun saiga-12 and Saiga rifles in rifle calibres both are centre fire rounds that cycle the same way using the same mechanism. The Saiga-12 has a larger ejection port, but look at the drawings of the ejection port for the AK12. The magazine size is also different and obviously the bolt face will need to be different to handle the different cartridge cases... but those are the same problems you have to deal when converting the action between different rifle calibres.
AK shotguns are quite different mechanically.
Actually, no, they are not. A shotgun fires a very large mass or normally smaller projectiles at relatively low speed, while a rifle fires a smaller lighter projectile at much higher speeds, but in terms of recoil and energy they are actually rather similar.
Technically speaking, I think you could get away with the AK-12 in .308 being convertable to all common rifle calibers, it's just the rifle would be oversized when it's in the smaller calibers.
Technically you probably could, but as you mention it would not be efficient to do so as you would end up with a heavy overbuilt rifle if you tried to make it an every calibre weapon including full power battle rifle capable.
I rather suspect they would have two rifle types... a light rifle for assault rifle calibres and shotgun calibres and a heavy rifle for battle rifle calibres and heavy shotgun calibres.
Examples of light rifle and shotgun calibres would include 7.62 x 39mm, 5.45 x 39mm, 5.56 x 45mm, Grendel, and that new US 6.8mm round, plus perhaps the new Chinese calibre too, and in terms of shotgun calibres for sporting use I would go for 20 gauge, and 12 gauge.
For the heavy rifle they have only mentioned 7.62 x 51mm for the military model and I have speculated on a 6 x 49mm or new replacement for the 7.62 x 54mm, but for the civilian model I would expect the 7.62 x 54mm plus the 30-06 and probably 12 gauge with 3 1/2 inch shells and of course a 10 gauge option perhaps too.
The question is... will the heavy rifle be heavy enough to go to .300LM and .338LM and perhaps the 12.7 x 55mm rounds they are talking about.
Changable barrels would be nice, but it's not something I'm crossing my fingers for.
For a civilian hunter the advantage of different calibres is that they just need one weapon that can perform a wider range of tasks. A multi calibre rifle will be more expensive than a standard rifle, but a multi calibre rifle with 4 calibre sets will be cheaper than the 4 rifles you would need to get the same job done.
I suspect at some stage a very light rifle might be developed with .22lr and .22wmr and of course .17wmr, plus 410 shotgun options.
For special forces it means learning to handle one weapon... only needing to buy one set of accessories like front grips and lights and sights etc and having one easy to use rifle for a range of roles without needing several different weapons.
Assuming the AK-12 just has Yugo type raised mag followers, you would have your BHO there. To have a true BHO and bolt catch in the AK-12, it would be kinda complicated. I really think it would be a big problem to have two types of AK-12's on the civilian market, one convertable, and one not convertable. They should try to push for the convertable one.
When it comes to selling to civilian customers it makes no sense to standardise into a one size fits all... different customers will want different things... for instance here in NZ we will want a standard rifle stock design with no free standing pistol grip. For our market not having such an option will greatly limit the number of people prepared to buy it. For other markets it might not be an issue, but having the different types means better options for the customer which is a good thing.
Considering the location of the button for the BHO device I rather suspect they probably went for a design that uses standard mags with a pin or wedge that projects forward from behind into the rear of the magazine. If a fresh round is present the wedge or pin is blocked which allows the bolt carrier to come forward and load a fresh round from the mag. If there is no ammo in the mag then the pin or wedge can move forward and the bolt carrier is blocked which holds it open when the last shot is fired or the mechanism is racked back on an empty magazine. Not rocket science or complicated... like I said the SKS already had that feature in 1945.
I rather suspect that for the most part the fixed, non-modular AK12s will sell rather well as they will be simpler and cheaper than the multi calibre models. This means for units of the Russian military who only ever use 5.45mm calibre ammo it makes no sense to go for a multi calibre modular weapon. For the average grunt the AK12 is easier to use and better than a standard AK. For special forces however the extra cost of a modular weapon is justified and will save them money by reducing the actual number of weapons they need to buy and own.
For a civilian it comes down to choice... for some they will buy one calibre and not bother with the other calibre options. For others they will take advantage of the more expensive multi calibre model and save money by buying several calibre kits instead of several different calibre rifles.