I mostly spend my BfA war resources on missions from my mission table. I used to prioritize azerite missions, but now that I unlocked all my slots I do the gold ones and the rep ones to speed progress to paragon chests.

Not 100% sure about the mage, but they could have had a soulstone on them. Warlocks can put a soulstone on anyone who can then use it to revive once as long as the buff lasts.

The flying mounts when dead is an automatic thing that only certain zones have. I don't get it in Northrend on my alt, but I do get it in Uldum and Vale of Eternal Blossoms on my main. I think you get it in BfA zones once you've unlocked flying.

The war campaign and yellow quests award rep, which can help with allied races and BfA Pathfinder (flying).