True Xehanort never liked Ventus because he was "unfit" like you say. However if Xehanort had a change of heart it's possible that he has the power to get through Castle Oblivion's defenses. He is stronger than Aqua in magic. Also there must of have been some sort of purpose for Xemnas to search for the CoW. Unless they plan to use Ventus as a hostage.
He speaks pretty clear in the reports about how much "potential" Ven has but that he's too benign for his own good and that his body is "too frail" to serve as a vessel.
Even if he had a change of heart it wouldn't help him anything. Finding the chamber in Castle Oblivion has nothing to do with magical prowess but who used the mechanism.
Aqua used the mechanism and thus she's the only one who can find and reach the chamber where Ven's body is.
Even if Xehanort would dress in drag and dance the hula he won't get in there ever if Aqua doesn't allow it, and considering how her opinion toward him is I guess the chance of her driving a Keyblade through his chest is bigger than anything else.
The purpose as to why Xemnas searched for the chamber is right now unknown. The only things that we do know about it are that Xemnas was apparently very eager to find the chamber. It may have been Xehanort- or Terra-influenced, maybe even both of it at the same time.
When looking at Days and some conversations between Axel and Saix, it seems that Saix was also quite keen on finding the chamber himself, although
his reasons are even more obscure except some possibility of him planning to use whatever they find in the chamber as leverage over Xemnas:
Days said:
Saïx: The castle hasn't given up all its secrets. And there's one in particular Lord Xemnas is especially interested in.
Axel: You mean the chamber. We turned that place inside out. If it's there, we're not gonna find it by just looking.
Saïx: Hmm... Then let me give you another incentive. Naminé isn't the only one who can trace her beginnings to Castle Oblivion. Xion comes from there, too.
Axel: Huh?
Saïx: Two of your favorite people. Still think another visit is a waste of time?
Axel: Hmph... Whatever. You're the one who wants me to go. Don't try and dress it up as a favor.
Saïx: Hmph.
Axel: Let me guess... That chamber will tell you everything you don't know about Xemnas's true agenda. Is that the idea?
Saïx: The Chamber of Repose, and it's antipode, the Chamber of Waking... Xemnas isn't telling us something. And the missing chamber, the Chamber of Waking--it holds all the answers. We need those answers to gain the upper hand--for our own reasons.
I honestly wished that Ventus could have had at least one shot to beat the crap out of Xehanort, lol
He would certainly deserve a shot for sure, as he was the first victim of Xehanort (that we know of so far) and had his heart nearly destroyed
twice because of him he clearly has as much right as Terra has to give the old goon a piece of his mind and more.
Best solution for that would be probably having the final battle Sora against Xehanort with Terra and Ven as party members.