Time for me to chime in again.
On the subject of WoHS/Knights as a core vs prestige class:
My big problem with having them as core classes is the fact that they're campaign-specific modifications of a core class. I do not like this. It's against the flavor of 3e, IMO. In 3e, the core classes are generic, i.e. anyone can become one. The prestige classes are supposed to be campaign specific groupings. Core classes should not be modified, IMO.
If it means that much to you that the WoHS always take the test at level 3 (I think this was an artificial constraint created by the rules set available in 1e, not a setting flavor item -- certainly not something that's worth keeping IMO), and you want the character to advance further than the default max PrC level of 10, then make the WoHS a 17 level prestige class.
Above all:
do not modify the core classes!
As for Sorcerer...
When 3e first came out, I instantly thought the Sorcerer was designed to help fold the SAGA magic rules back into D&D. Even though I have it on authority that this was not the reason for the Sorcerer (RyanD), I think it might be a good use for it. Although there would probably have to be a class "name change" to keep people from confusing the class with High Sorcery. Perhaps call it Magicka, or something.
Anyway, pre 5th-age, Sorcerers do not exist, or if they do, they are unheard of and extremely rare (there's no reason why Palin and Goldmoon have to be the first to discover the power) -- they keep away from the watchful eyes of the High Sorcerers. Although modifying the spell-list might be an idea to keep the Sorcerers in line with the feel of SAGA magic, it might require extensive additions to the spell list as well -- something that might be a problem. Because of this, I suggest that, rather than changing the rules to fit the setting, change the setting to fit the rules. Probably a bad idea, but a good fix on a system that never should have existed at all, IMO. Of course, this part may be moot after June -- Weiss/Hickman seem to be planning to ret-conn the entire 5th age out of existence, but we won't know for sure till the book series is done.
If that ends up the case, I suggest that rather than even try to adapt 5th age to D&D, they should just say "Go play the SAGA version if you want to play in this alternate time-line that ended up never happening". Sorcerers, under such circumstances, could become hedge-wizards, witches, renegade spellcasters. Outcast, always on the run, never able to show their powers, etc.
On the Irda as Sorcerers.... I think this has potential, but I'd rather see the Irda represented as Psionicists. They always seemed a bit psionic to me in DoSF -- loners, yet hive mentality, wierd magic with different effects than "conventional" magic, etc. The Wizard/Sorcerer spell list is identical, Sorcerers are just Wizards who happen to know how to cast their spells without having to prepare them first.