Hello,
Yes you can gap just the center leg, but that's more difficult for a custom design. It's much simpler to run a piece of insulation across the flats of the whole thing. Yes, the effective gap is then twice as much as the thickness of the insulation so you'd then have to go with 1mm if you wanted 2mm for example. Yes better shielding with only the center leg gapped.
The maximum gap really depends on the number of turns on the primary and the primary voltage as well, not just the core size. Together these things must be able to support the required primary voltage in normal operation.