I'm not sure about your theory on power picking. It is certainly a precise reading of the rule, but I've also seen statements that appear to contradict that interpretation.
Random people on the internet don't count as a printed rules source. I've seen statements online that claim Combat Challenge is an opportunity attack, that doesn't even make that sensible.
The point is, sun clerics cannot pick storm powers. There's a distinction there, it tells you -exactly- what powers you get, and what powers those replace.
Example: From Level 17
'You gain an encounter attack power associated with your domain power (storm, page 109, or sun, page 115). This new power replaces the encounter power you gained at 3rd level from your domain.'
This is -very- explicit. You get the one power written on that page, and it replaces no other power but the one you gained at 3rd.
There's NO choosing of a power involved, no more than a Bear Shaman can choose anything but Spirit Shield, or that a Fey Warlock chooses Eyebite.
Oh, hey, Eyebite has a level involved too... why would that be?
Those may be mistakes though, you can't always trust various blurbs by devs online. The only thing that goes against you a bit there is the question of WHY a power that is strictly presented as a class feature and immutable would have a level and be labeled as a class power instead of the more standard "feature" designation that has been generally used in the past.
This precedent already exists. Those powers are selectable by any character that has the eligibility to select powers of that class, type, and level. A hybrid Avenger/Cleric is allowed to take warpriest powers because he has the right to pick them. A human laser cleric can take one of the at-wills because he has the right to choose any cleric at-will power. Every non-warpriest cleric can choose one of the domain powers because of they are cleric powers with the appropriate type and level.
This precedent was originally set by human warlocks and half-elf paladin/warlocks-- humans have the ability to select their third at-will, and that at-will can be any warlock at-will, even tho their other two are decided for them by pact. (exactly like a warpriest)
Shamans are the same way, one of their at-will powers is selected for them by their spirit boon. They simply can't switch it out for another at-will power.
Why are essentials characters, who are even MORE rigid in power selection, suddenly ignoring this precedent? Where is it written they get to do that?
It isn't.
I think we'll have to see an FAQ or something like that before it is actually clear what was intended.
I don't think you need a FAQ to institute a rule that's been in existance since the PHB1 and has never changed. Unless you have a specific exception, if your class feature tells you to take a specific power, you take that specific power. Even if that power has a level.