Hmm... Good question. I'd say there could be up to 2 parts, but I don't know how many episodes in each. Tales Of Arcadia is probably going to be releasing episodes every six months, so 3Below will be released in Nov/Dec 2018, and maybe May/June 2019. 3Below can't release a 3rd part because Wizards is being released in 2019, which is probably going to be released in Nov/Dec 2019 (or earlier if 3Below doesn't have a second part).
And, I think Krel and Aja will have to eventually reveal their alien form, probably to the Creepslayerz, someway through the show. It will probably be the early part of it if the Creepslayerz are gonna be allies with the aliens, and see all the alien tech/villains.
Probably not. This is all just speculation, but I think after the Creepslayerz gain the aliens' trust, or if the Creepslayerz are attacked by aliens, Aja and Krel will be forced to reveal their true identity.
As explained in one interview with EW, Aja and Krel's mothership gives them their disguise. How it works, I don't know, but their self-aware mothership, who was the one who flew them to Earth, was the one who chose the kids' human forms.