We need to stop ragging on about the strength of light or the strength of the darkness within the heart to determine whether or not a person would survive the splitting process. Yes, Ventus had a lot of light in his heart, yes, Aqua also has a lot of light in her heart, but when MX separated the darkness from Ventus that was when he was, what, ten, eleven? The strength of his HEART ITSELF was weak at the time. He had a Keyblade, so he must have had a strong heart, but it was comparatively weaker to the strength of Aqua's heart at the time of BBS, about five years later or so. Aqua's heart itself was very strong, so even without the darkness she has a much higher probability of surviving without connecting her heart to someone else's.
This is actually the statement which comes near the truth the most.
Xehanort couldn't/did not want to use Ven as his new vessel because he was too frail both physically
and had too much light.
In the X-Reports it was also stated that Ven is to benign and kind for his own good, indicating a very strong light in his heart.
Xehanort tried to make Ven use the little darkness he had, but the kid outright refused to do it.
As said, Ven was 9/10 years old at the time of the neo-shadow attack and him being attacked by the heartless probably additionally weakened him.
When MX did the heart-cracker Vanitas took much of Ven's power at his creation...and started to absorb/leech off even more power from the injured kid...eventually sending his injured heart into a critical condition.
So not the initial "heart-split" was the reason Ventus started to "die" but because Vanitas took too much away from him.
Between the time of Vanitas's creation and MX dumping Ventus on DI there is a gap of some days until MX realized that Vanitas was literally devouring Ven, as he initially planned on training both boys to be his x-blade material, as indicated in Xehanort's Report 10.
So Ventus was already severely weakened even before Xehanort performed that cruel heart-splitting attack, making it easier for Vanitas with his new dark heart to send Ven into a critical condition.
As for Aqua, she was trained since early age by Master Eraqus and that training has made her heart filled with enough light to be a suitable x-blade material.
That's also what Nomura seems to have meant.
Actually it was Xehanort's initial plan to turn Ventus into a dark warrior and pit him against one of Eraqus's light students to get the x-blade, yet Ven's kind nature and refusal to use his own darkness prevented that and MX, getting pissed over this resorted to creating Vanitas.
Taking this all into consideration I think we either may not take this "pure" in a literal sense or there is a way to make a heart "pure" without actually destroying it.
(Which may in Ven's case also a combination of him already weakened by battle, him still a kid that time, Vanitas feeding off his power and not to forget the bunch of heartless that overwhelmed him.)