Sensei from the other timeline was severely disfigured, and the events of our timeline happened without his guidance; without him to guide his students, everything goes horribly wrong which leads to the deaths of almost everyone we got to know in our timeline. Alt-Sensei makes a deal with the same beings that destroyed his Kivotos to become their vanguard, becoming the creature you see here called Phrenapates.
@GreenTea💚 Alt Sensei made a deal to be some sort of Prophet to this being, and use it's power to jump into other timelines, originally Shiroko was supposed to be this prophet but Sensei took responsibilty you beat him and you managed to Save Shiroko terror, it is heavily implied that Sensei's desire to save his students still lived withing him, and you saving Shiroko terror was his actual plan all along
The idea of SpongeBob being a resident of Kivotos is strange, yet somewhat possible since all the other residents that aren't humanoid like the students are anthro animals, robots, or some other third thing that's basically a miscellaneous group of every other being living in this city. One issue with that possibly not being true is that he will dry up in Sandy's treedome due to not being exposed to water and needing to have a fishbowl helmet with water to be in it (Tea at the Treedome), however this isn't the case in scenarios like the first movie, as both he and Patrick were on land. Maybe the dome magnifies the sun's heat like a magnifying glass or conducts it faster.
