Lord Kroak

In Starborne, the Slann Starmaster is a must-take wizard.  It has excellent abilities, a phenomenal spell lore (and it can swap out the spells as a Heroic Action), top-notch command abilities and access to one of the best artefacts in the game.

Comet's Call in the Lore of Celestial Domination can deal D3 mortal wounds to up to six enemy units anywhere on the battlefield, making it a must-cast most turns.

The command trait known as the Lord of Celestial Resonance will typically add 3 or more Cosmic Power Points to your Cosmic Power pool each turn, which means more summoning or other Cosmic Power abilities.  Your Starmaster will need to be your general to take a command trait.

The Spacefolder's Stave is an artefact that you can build your strategy around.  The ability to teleport, summon or beam down (in Dracothion's Tail) your units just 7" from the enemy means that units can fairly reliably charge immediately upon arriving (especially since many of our units get +1 to Charge).

In Coalesced, it isn't so clear that the Slann Starmaster is worth the 290 points.  It has the same great warscroll, which will benefit any army, but most of the spells of Lore of Ancient Domains aren't fantastic, and the command traits and artefacts aren't amazing either.  Still, there has been a growing trend to include the Slann Starmaster in Coalesced lists, often taking the Shaman of the Chilled Lands command trait from the General's Handbook to give the Slann access to all the spells of the Lore of Primal Frost.  

No, not at the same time.  When taken in Starborne, Lord Kroak knows all the spells in the Starborne Slann spell lore, the Lore of Celestial Domination.  When taken in Coalesced, he knows all the spells in the Coalesced Slann spell lore, the Lore of Ancient Domains.  He never has access to all nine spells in a single army.

No.  Only Andtorian Loci can take spells from the Lore of Primal Frost, and unique characters and models with more than 9 wounds can't be an Andtorian Locus.

No.  Core rule 27.3.1, Enhancement Restrictions, states that Unique units cannot take enhancements, which includes the Universal Spell Lore.

No.  Core rule 27.3.1, Enhancement Restrictions, states that Unique units cannot take enhancements, which includes command traits.  For this reason, Lord Kroak should not be chosen as your general.

No.  Core rule 27.3.1, Enhancement Restrictions, states that Unique units cannot take enhancements, which includes artefacts of power.

Yes.  Lord Kroak is a Warmaster.  This means that even if he was not picked to be your general (and he should not be):

• Lord Kroak can issue commands at a general's 18" range.
• If your chosen General is slain and Lord Kroak is still on the battlefield, you still get your command point for having a general in each hero phase.
• If Lord Kroak is slain, you get +2 to Heroic Leadership rolls even if your picked general is still on the battlefield.
• You can complete battle tactics and grand strategies that refer to your general with Lord Kroak.  However, if these refer to "the unit picked to be your general", they only apply to Lord Kroak if he's your army's chosen general.

Lord Kroak's 'Supreme Master of Order' ability says that it can unbind spells or dispel endless spells "anywhere on the battlefield".  The question has come up as to whether visibility is still required.

For unbinding, the answer is clear.  Unbinding never requires visibility, so Lord Kroak can unbind spells cast literally anywhere; there's no escaping the magical reach of this powerful Relic Priest.

Dispelling normally does require visibility, however.  The official world championship Tournament Organizers have stepped in and made a ruling on this question, looking not just at Lord Kroak's warscroll, but all the similar rules throughout the game.  They concluded that while the wording on Lord Kroak implies visibility should not be required, that ruling would create messy inconsistencies with other rules throughout the game, so it was decided that "anywhere on the battlefield" would remove just the normal range requirement of a rule and not the visibility requirement.  With that ruling, unless Games Workshop steps in with a FAQ or Errata, visibility is still required to dispel an endless spell.

No