Focus of Mystery spells

No. Unless specifically stated otherwise, normal rules apply. The normal rule here is BRB p. 72: "A Wizard with the Loremaster special rule knows all the spells from his chosen lore - he does not need to roll randomly. The lore in question is normally given in brackets as part of the Loremaster special rule. For example, a model with Loremaster (Fire) would know all the spells from the Lore of Fire."

In accordance with that, the Focus of Mystery states that "the Slann Mage-Priest has the Loremaster (Lore of High Magic) special rule." That is his chosen lore - he cannot choose another one. That is precisely why the Wandering Deliberations and Focus of Mystery rules specify that they are mutually exclusive.

I would say all Wizards will benefit, as is indicated in the description of the Sphynx (BRB p. 130): "If the test is passed, the challenger receives one of the following special rules, determined randomly, for the rest of the game: Devastating Charge, Heroic Killing Blow or Loremaster (Death) - this last one has no benefit to non-Wizards (i.e. only Wizards can actually use the spells!) but that doesn't stop the Sphynx bestowing it inappropriately." 

That is the difference between list building and battlefield. Requirements for building a legal army list can be and usually are no longer applicable, once the battle has started. When you build your list, your Wizard is allowed only one chosen lore (unless specifically stated otherwise, of course). That does not mean a Wizard could not gain another lore he has not chosen, because of conditions on the battlefield - just as a Wizard even a Loremaster, can lose spells. 

As it happens, something similar was discussed last year around this time.

I don’t remember the specific wording, but End Times Khaine magic rules effectively gives Loremaster to all wizards, and I think the example mentions the High Elf Loremaster, who basically has Wandering Deliberations and it explains that he would have Loremaster for all 8 lores. The example may have used a different type of Wizard that can have spells from multiple lores in game (like a Skaven grey seer or kairos fateweaver, but not an empire wizard who can just choose multiple lores)

So, there is some basis to show how a wizard tower could work for a wizard with spells from multiple lores.

They changed the Loremaster rule in End Times because they basically gave all wizards pre-End Times Loremaster.

And the End Times example specifically mentions how pre-end time Loremaster works for a High Elf Loremaster (who gains spells in the same way as a Slann with Wandering Deliberations).

If you can’t see how that gives an example of a Wizard Tower interacting with Wandering Deliberations, then you are being deliberately obtuse.