From my understanding, you need a strong heart to be able to wield the Keyblade. Like Seighart said, I'm not sure if the Inheritance Ceremony is totally required for you to have it. Take a look at Vanitas. To our knowledge, he was not chosen to wield the Keyblade (unless it was performed by MX off-screen) and was able to due to his strong heart (albeit one of pure darkness). If it is to be totally necessary, then I would put a gamble on Yen Sid or Mickey [under Yen Sid's order] being one of the people to preform the Inheritance Ceremony for Lea due to noticing his strong heart.
The inheritance ceremony is a given prerequisite according to Nomura. Sora is the only
person ever to wield the Keyblade without having gone through the proper ceremony, but that is now easily explainable due to his strong connection to Ventus.
And before someone (I don't mean you specifically) comes around again and fears that Sora might lose the Keyblade after Ventus wakes up: NOPE! Sora can still wield as long as the connection
with Ven's heart remains.
It was not
Sora allowing Ven's heart to sleep inside his own that granted Sora the ability to wield. Ven's heart going into Sora only allowed the dual-wielding.
Sora's heart was originally not suited/meant to be a heart to wield, but that changed already several moments after Sora was born because his newborn heart connected
to Ven's heart to save it. That
gave Sora's heart also access to the ability to wield a Keyblade (like a sort of basic, direct heart-to-heart-ceremony) due to how closely his heart is knit together with Ven's.
So even if Ven awakens and his heart leaves Sora, Sora will still be able to wield as long as the connection between him and Ven is not severed, which I can't imagine anyone of them both wanting.
Furthermore, the Keyblade itself (Riku's former one) choose Sora, not Ventus inside Sora (Ven already has a Keyblade, he doesn't need another), so it is even very much possible that after all this time and use of the Keyblade Sora's heart is by now itself fully qualified to wield.
Vanitas is a bad example though, as Vanitas' dark heart was originally born from Ven's heart. At the core of Vanitas' heart there is a shard of Ven's original heart which then grew (that was probably MX' doing, just like Terra, Aqua and Eraqus helped re-form Ven's damaged heart) into Vanitas's fully fledged dark heart.
Since he came originally from Ventus though, just like Roxas in regards to Sora, Vanitas can wield without any problems.
As far as we know, under normal circumstances, you'd need to have a keyblade ceremony to wield. Only special cases permit you to wield without having to go through it, to which it's possible that Lea was a special case.
I certainly don't see what's the fuss about that.
Why don't you just all use the most obvious choice instead of making Lea more special than he actually is. Sora is the only one to wield without a "proper" ceremony.
Lea came and asked Yen Sid to become a wielder, so the most easy and logical answer is that either Mickey or Yen Sid himself performed a Keyblade inheritance ceremony with Lea.