Fellowship units can be reborn as Manes, even Order units could be. Any unit which knows Death 1 or Entropy 1 can get drafted by Hyborem upon death.
2. There is an event you get from shipwrecks that says something to the effect of you find a sea serpent that turns on you. No sea serpent. I guess this might have been mentioned before with the lack of sea-going critters. I did manage to get a pack of Drowns from the Isle of Aifon though.
Which animals are HN?
Boar riders can't be built because they require stirrups, stirrups require trade, chariots are enabled by trade, and chariots obsolete boar riders.
Sea Serpents are invisible now, due to the existence of the Jotnar. So one probably spawned, you just got lucky and it didn't attack you.
That reminds me of another minor irritation - after building Nature's Revolt, HN animals will enter your borders and attack you. It seems kind of against the flavor of that ritual.
Why would Chariots obsolete boar riders? In most other civs you don't have Chariots obsoleting Horse Archers.
If that is the reason, then I suggest it be changed as it is removing a UU from the dwarves' repertoire.
Also, I just wanted to see what that unit model looks like!