Marrina may not have morphed "all over the place" under Byrne, but she did exhibit slight biological changes. The Plodex race itself could adapt to its surroundings, such as Marrina taking human form. The Plodex in Lake Ontario also seemed to be metamorphing. To me, Byrne opened the door to the possibility with those things. Some post Byrne changes make perfect sense.
I love almost all of Byrne’s work, but especially on his boards he is more concerned with being right than in making correct statements.
There is common sense to "why run fast when you can fly faster?" But there is an answer: space. Byrne said "Northstar was just flying close to the ground" in X-Men 121. Didn’t look that way to me. For an Alpha citation of superspeed running by Byrne, how fast did Aurora change clothes in that scene in Walt’s apartment? Should we think she changed clothes while flying indoors? Makes me wonder just how SLOWLY they can fly to stay airborn.
I think there’s also a shared core of powers to Northstar and Aurora, so I never minded any of the extra abilities Auroradeveloped with the light powers. In AF#1, Byrne established that the light powers can affect the Ems of Guardian’s suit. That opens the door to there being a lot more to the light powers than Byrne explored. They may be twins, but they seemed to develop their powers slightly differently. Aurora developed individual light powers after Byrne’s "mutant alteration" storyline, so it is natural that she could do more varied things with the light.