There are a lot of high CR threats in this book. With 44 tier 3 and 27 tier 4 monsters respectively, it's just amazing how much space in this book is devoted to monsters that the vast majority of people will never actually use. Most campaigns of D&D end somewhere around level 11, and even the ones who go higher are likely to focus on a handful of these end game baddies.
Bael, being a legendary CR 19 Archdevil, is an endgame threat that your party isn’t likely to face anytime near the start of the game.
Yes, that is more useful. Various features key off one or the other. One of the things that key off "weapon attacks" (though weirdly not all) involve the use of the tools that are known as "weapons". One of the things that key off "spell attacks" (though not all) is the use of the features that are known as "spells".
I’m not even going to pretend like anyone knows what this name is supposed to be. I’m sure it must be some kind of ancient dwarven word that means “fire” though. Seeing as how they are Flamethrower units.
Bless is a spell that can be countered, but bardic inspiration cannot, even though the latter is magical as well. Countercharm is magic, but it's not a spell like many other protective spells, and thus it can't be counterspelled. Performance of creation from the College of Creation subclass is a very magical ability that creates something out of nothing, but it can't be counterspelled as it's not a spell. There is a spell called creation however.
Channel divinity is a magical ability that is not a spell. It can't be countered. It's just as magical and comes from the divine as the cleric's or the paladin's spells. Wrath of the Storm is a magical reaction for the Tempest domain, but it's not a spell and can't be countered. The warlock spell hellish rebuke however is a reaction spell that can be countered.
Wild shape is a magical ability that can't be countered. Polymorph and the other shapeshifting spells can be countered. Spirit totem is a magical ability of the Circle of the Shepherd subclass that summons a spirit, but it can't be counterspelled unlike spells that summon things.
You get the point? We could go on and on. 5e has been selective and inconsistent about counterspelling from the very beginning. It was never logical.