There was a brief time in which it's possible that western Virginia could have simply claimed to be Virginia- that they'd never seceded. But pretty soon after deliberation in the Wheeling Conventions it was decided to create a new state. For that, the essential Constitutional phrase was "without the consent of the legislatures of the states". The legislature of Virginia had to consent. Tricky thing was, who was that legislature? There was the recent "loyal" one in Wheeling, and the Confederate one in Richmond. Both had been elected by Virginia voters. Lincoln's cabinet was equally divided on admitting West Virginia, and on [Dec. 31, 1862 he wrote his own very clear opinion on the subject] ( https://www.loc.gov/resource/mal.2062000/?st=text) :

The consent of the Legislature of Virginia is constitutionally necessary to the bill for the admission of West-Virginia becoming a law. A body claiming to be such Legislature [in Wheeling] has given its consent. We can not well deny that it is such, unless we do so upon the outside knowledge that the body was chosen at elections, in which a majority of the qualified voters of Virginia did not participate. But it is a universal practice in the popular elections in all these states, to give no legal consideration whatever to those who do not choose to vote, as against the effect of the votes of those, who do choose to vote. Hence it is not the qualified voters, but the qualified voters, who choose to vote, that constitute the political power of the State. Much less then to non-voters, should any consideration be given to those who did not vote, in this case : because it is also matter of outside knowledge, that they were not merely neglectful of their rights under, and duty to, this government, but were also engaged in open rebellion against it. Doubtless among these non-voters were some Union men whose voices were smothered by the more numerous secessionists; but we know too little of their number to assign them any appreciable value.2 Can this government stand, if it indulges Constitutional constructions by which men in open rebellion against it, are to be accounted, man for man, the equals of those who maintain their loyalty to it? Are they to be accounted even better citizens, and more worthy of consideration, than those who merely neglect to vote? If so, their treason against the Constitution, enhances their Constitutional value! Without braving these absurd conclusions, we can not deny that the body which consents to the admission of West-Virginia, is the Legislature of Virginia.

He then stated that it's another question as to whether it's "expedient": i.e. a good thing to do. That, Congress had to decide. But for his own opinion:

The division of a State is dreaded as a precedent. But a measure made expedient by a war, is no precedent for times of peace. It is said that the admission of West-Virginia, is secession, and tolerated only because it is our secession. Well, if we call it by that name, there is still difference enough between secession against the Constitution, and secession in favor of the Constitution. I believe the admission of West-Virginia into the Union is expedient.

It would not be until 1864 that Congress would also finally decide it was "expedient".

While we wait for someone to provide us with information on any legal challenges to West Virginia's statehood, you can in the meantime check out this answer from u/Bodark43 that provides some relevant details to the event