A) They're from Default half-life. Use GCF-scape to extract the models from half-life.gcf in your Steamapps directory, then use the entity monster_furniture or monster_generic and choose the appropriate model.
B) There's always the
HIT forums, they have loads of fly models.
C) No. Spirit rules, and yes, it's used as a mod base.
Orpheus, lots of popular mods are based on Spirit... at least DoD is, and that should be incentive enough.
Honestly, it's great to base a mod on, gives you lotsa more options.
D) Ambient_generics aren't only for ambience sounds - ambient_generic looks in valve/sounds by default, not valve/sounds/ambience - if your sound is valve/sounds/hello.wav, just type hello.wav in the ambient_generic's options.
And a note - hardly anyone uses WON-HL these days - Files are packed in PAKs.