Cleric 3/Diviner(scryer) 1/Mystic Theurge 10 is very strong for someone who wants to focus on spellcasting instead of channeling. The only thing non-core is using a subschool from the APG, but if they deny that you might as well play straight Cleric because nothing that makes you more powerful is gonna fly. The best part is that this doesn't restrict you to a specific race, rely on traits, rely on domains or rely on feats. You can build your cleric however you want and you end up with level 9 divine spells at level 18 like an oracle. You give up good channeling for level 6 wizard spells and spell synthesis. There is going to be an issue of wanting both armor and wizard spells in combat, but the armor will eventually be replaced by wizard spells so this issue is only really there at mid level.
The SLA that replaces your first level school power mimics Clairaudience/Clairvoyance and counts as being able to cast 3rd level arcane spells. There is a whole guide for early entry into prestige classes using SLA's.
EDIT: Actually it might fall to an odd technicality where you can cast 1st and 3rd level spells but not 2nd, ask in a rules thread or talk to your DM.
Yeah, to fulfill 2nd level spell requirements, you actually need a 2nd level spell/SLA.
That is consistently mentioned whenever the subject is brought up, including by Paizo themself.
As mentioned, you can get the divine side by domain, and possibly a few racial SLAs (and/or Feats expanding racial SLAs), and there are more options than just Aasimar, I think there may even be a few more beyond what was mentioned. That list seems mostly to be races that would also be banned in a game where Aasimar are banned, but Half-Elves are a pretty standard thing. (I believe it requires an Alt-Racial trait, but probably more likely to be allowed)
Focusing on Channel to that extent from Level 1, when Mystic Theurge will never advance it, is just absurd.
Selective Channel is silly enough as-is, Channel isn't the hottest in-combat action you can do to begin with, and since Channel (like Casting) doesn't depend on Full-Round Actions, you can just spend your Move Action to include/remove targets you want/don't want from the AoE.
Regardless, you will be better off NOT focusing on Channel like that,
but taking the Human Alt-Racial (replacing Human Bonus Feat) that gives two +2s to different stats.
That allows you to split stats easier, making it so other options besides Empyreal Sorceror are good options.
Although honestly you don't really NEED both arcane and divine sides to have awesome DCs,
just choose one for that, and one that will be used for DC-less/buffing/utility spells.
You are probably better off going with Cleric+Wizard instead of using Sorceror, CHA synergy between Sorc and Channel just will not matter in the long run because Channel won't be advancing in Mystic Theurge, and having both arcane/divine be 1/2 a spell level ahead (from being prepared casters) is a further advantage: that's giving you a further +1/2 DC advantage to go with the more powerful spells.