I can ask, but what I assume is that the Betty was 15,000 lbs (empty) on 3,060 horsepower while the B-25B's used on the Doolittle Raid were 19,480lbs (empty) on 2,700lbs horsepower. Betty had a better horsepower to weight ratio. (The B-25C had upgraded engines but I checked Wiki, and the B model was used).
And this brings up a serious problem in IJN bomber design philosophy in WW2. The Betty was actually a honey of a bomber in potential if they'd armored it properly and forgotten about trying to eek every ounce of long range out of it.
Dunno about the full-up weight issue. I don't think the OP's proposal can work because the IJN isn't sailing a carrier that close to the West Coast.Also, the question is not whether or not a G4M1 could take off from a carrier with "a reasonable fuel and bomb load". The "what if" proposes that each G4M1 would carry a full load of fuel and bombs, and Francillon puts its loaded weight at 9500 kg (about 21,000 lb). The G4M1 could not be lightened by removing armour, since it had none, so probably the only way to lighten it would be to remove some or all of the defensive armament plus the associated gunners and ammo.