Yes, I settled on “reserves” meaning “keeps back” to justify it, rightly or wrongly.
I cold-solved the entire puzzle before attempting the gridfill. Out of interest, muraria, did your final grid contain jumbles, even though those galaxies retained the thematic shape of the others? It sounds like I pretty much followed your approach, ignoring the first half of the preamble.
I think when you’ve completed it the orientation will make logical sense. As it’s a prize puzzle, I don’t want to give too much away on a public forum, but if you email me at patmark291atyahoodotcom I’ll send my alternative preamble