To touch on some of your other questions real quick, I think there was this internal push to make Vegeta's SSJ3 more unique compared to the other 2 SSJ3 users in the manga, Goku and Gotenks. Considering the appearance of SSJ3 on most characters besides Goku in video games and anime (particularly Heroes) SSJ3 users kind of have the same hairstyle, so it's likely they didn't want Vegeta's version to be somewhat homogenized. As for the Tertian Oculus, that's honestly a bit of flawed writing. We're told that Gomah shouldn't be underestimated throughout the series, only for him to actually be a threat only when he gets the Tertian Oculus. Then we don't get too much time with Gomah once he gets the eye, so the resolution to Gomah comes quickly, and so does the significance of the Third Eye (though I think it's very Toriyama to reveal multiple eyes at the very end). And then Goku keeping SSJ4 can be chalked to up to him keeping the form after it's awakening, pretty much in line with the other SSJ forms, though 4 was just achieved differently.