The best situation for adding an ATS is when there already is an outside main disco and an inside panel that already has the neutrals and grounds separated. In this case, ground/neutral bond in the existing main can stay, and the bond in the ATS is removed. You don't even have to pull the meter. You can simply nipple the ATS to the main like I did here:
Before and after; note that the bonding jumper in the ATS is removed, and the ATS is wired, like the existing panel is, as a sub-panel. I used straight set-screw butt splices to extend the original load conductors into the house. I didn't have to touch the original neutrals, and the neutral jumper to (or from) the ATS only needed to be large enough for the generator:
View attachment 2551516 View attachment 2551517
Also note that the ATS is not the main disconnect, but the service-rated ATS that was in the package I bought was cheaper than a non-service-rated ATS would have been to substitute. There is no reason to remove a now-redundant existing main breaker when adding an ATS, whether in a panel or in a separate enclosure.
On another one I did recently, I replaced the existing outside main disconnect with the ATS, partly at the customer's request, partly because of space reasons, so I did have to pull the meter, and the ATS bonding jumper needed to remain. The ATS was nippled to the bottom of the meter in place of the original disconnect.
The worst situation is what you have: a single assembly with factory-installed meter-to-main wires, which we're not supposed to modify, although I personally agree with you that it ought to be allowed. Before doing anything, I would get the blessing from the inspector for what you propose: intercepting those wires to insert the ATS in the pathway.
As the ATS would now become the main, the bond should remain in place, which renders the breaker section a sub-panel. That means, if otherwise allowed, you'd need to install an insulated neutral bus if the existing one is permanently bonded to the enclosure, and move all of the neutrals there.
Guess what is supposed to happen with existing 3-wire appliance circuits: they're supposed to be converted to 4-wire circuits, as they're supplied from what are now sub-panels. As they're still fed from an enclosure with a ground-neutral bond, if the inspector let's it remain bonded, you might not have to do this.
That removable bond is to be removed when the transfer switch is wired in after a main disconnect. See above explanations.
No, that is not a stand-alone requirement. There are rules for when it is in the same enclosure, and rules for when it isn't Wherever the main disconnect is located is where the main bonding jumper is supposed to be, is where the grounding electrodes terminate, and is where the premises grounding system begins.
Well, if you add a "service breaker in a separate enclosure right next to the main", the new breaker is now "the main", and the existing main breaker becomes an allowable but not required redundant breaker. Because the service main is now in the new enclosure, the neutral must extend there, as well as the grounding electrode conductors.
As it would be used only as an EGC, the neutral jumper could theoretically be sized as small as a required EGC, and even be bare. However, as I said above, this is technically not feasible, as the meter enclosure can be bonded to the neutral, but a sub-panel enclosure can't be bonded to the neutral. You only have one enclosure; there's the rub.
