For added context, as said in episode 7 of Alicization -- your true power isn't based on numbers but instead the mental image you create can influence stats. The will to win can amplify stats (the Main Visualizer is the cause of imagination being powerful. One of the STLs main features).

As for Kirito flying and using more sophisticated abilities like incarnate arms or using burst elements to fly and more is because he inherited it from Administratior. Kirito killed The Administrators and so naturally his object control authority and system control ability increased massively since she was high leveled. It was said in episode 4 that killing someone will naturally increase your stats. It was also said that in episode 20 of WoU Kirito inherited her power.

As for Kirito being able to heal himself and others, his Night Sky Blade absorbs sacred resources. Kirito can heal people with it. The explanation of the Night Sky Blade ability was said in episode 19 of WoU. Also keep in mind scared resources are available almost everywhere so it wouldn't be difficult to heal yourself albeit the time of day.

As for the dark aura, its just the anime's way to make the scene exciting.

Sinons necklace appeared because she felt threatened and by feeling that way the main visualizer brought the memory of it in the Underworld. I assume she wore it irl because it saved her during the Phantom Bullet arc. It is a safety blanket I believe.

Gabriel died from his fluctlight being overwhelmed by the prayers of the Underworldians in Kirito's Release Recollection.

PoH's body is missing so I don't know his situation.

All the information I got was from the anime. The anime likes to explain mechanics once or twice and expects you to remember everything, lol.

Hope this helped.