4) I found that galaxies looked better with old sprite texture, so I don't change it.
Screenshots are taken with Skyboxes/Impostors/FB disabled (in Graphics settings menu). Open them in a new browser's tabs and compare by switching the tabs:
OLD galaxy and OLD nebula:
OLD galaxy and NEW nebula:
NEW galaxy and NEW nebula:
Glow becomes more irregular, but it virtually washed out dust details. First (old) screenshot looks better IMO...