There are more ways to skin a cat. The conventional layout of the Hornet has it's advantages - no new, unknown tech, easy installation of armament, in most of the cases easy additon of second crew meber, can be much lighter than twin boom - too bad Lockheed did not came out with P-38 in calssic layout.
The push-pull has it's advantages, that grew lesser with increase in engine power and approaching the 450-500 mph speed range, though they avoid the venturi effect that plagued P-38. What P-38 have had against Hornet and Do 335 was that it really was there when mattered, timing being a major thing for a wepon of war.
We can also recall that Hornet did it's job on next-gen aircraft engines, while Do 335 used 'legacy' engines for same turn of speed.
As a what-if, too bad Dornier did not went out earlier with such a concept, employing two Jumo 211 or DB 601/605 engines.
Re. 20 mm cannon ammo count for the P-38 - it was 60 rd drum up until -G, while the -H (1st delivery March 1943) got belt-fed cannons, 150 rpg.