KHH has more of an advantage over Masenko. If Gohan had it at the start then there'd be no reason to use Masenko. You'll almost never see an SSJ Gohan in a server using one.
Get a Piccolo and a Goku at the same powerlevel and fire a Masenko into a KHH, KHH is a stronger move, it would win the PS as long as neither/both sides held down primary fire.
ESF is not set at a specific part in the DBZ timeline, it would be impossible. So ESF compensates that by the characters needing to build up their powerlevels before they can trans'. Same applies for Gohan 'learning' KHH when he turns SSJ.
But rumour has it that Adult Gohan (with Mystic) will be added in a later (read: not v1.3) version of ESF, and he'll most likely have it as a starting move.
Yes it is, near the beggining of Dragonball, you should really watch the rest of Dragonball (DB) and then Dragonball Z (DBZ) to see that it's used as an offensive beam attack in fights. ESF is based of off DBZ where it happens a lot.
I hope that answers all of your questions.