Jason Young never offered an alibi to law enforcement. "Alibi" is a legal term for what a suspect tells LE, not what he says to a jury. Eight jurors found Jason's testimony to be credible.
If the prosecution wants a conviction, they must prove beyond a reasonable doubt that Jason Young had opportunity to commit the crime. So far, all they have provided is a brain-damaged witness who admits to memory problems and a random rock.
JMO
I understand the confusion now; you were thinking alibi does not mean when a defendant offers evidence through himself or someone else that it was not possible for him to commit the crime because he was somewhere else. Alibi is a legal term for a defense offered in court to a criminal charge. The defendant does not have to testify to his alibi, but JY did. It is also a story you can tell the police as you indicate, but that is not the crux of it.
"An alibi defense can be presented if the 'defendant was not present at the time when, or at the place where, the defendant is alleged to have committed the offense charged in the indictment.'1"
http://txw.fd.org/LinkClick.aspx?fileticket=dl3fls_DHKw=&tabid=58
See also as an example:
"Federal Rules of Criminal Procedure
RULE 12.1 NOTICE OF AN ALIBI DEFENSE
(a) Government's Request for Notice and Defendant's Response.
(1) Government's Request. An attorney for the government may request in writing that the defendant notify an attorney for the government of any intended alibi defense. The request must state the time, date, and place of the alleged offense.
(2) Defendant's Response. Within 14 days after the request, or at some other time the court sets, the defendant must serve written notice on an attorney for the government of any intended alibi defense. The defendant's notice must state:
(A) each specific place where the defendant claims to have been at the time of the alleged offense; and
(B) the name, address, and telephone number of each alibi witness on whom the defendant intends to rely."
http://www.law.cornell.edu/rules/frcrmp/rule_12-1
Of course, the lack of an alibi or even the presentation of an alibi that is not believable is not proof of guilt. I agree it is to the prosecution to meet that burden. Nevertheless, JY testified to his alibi of having been in VA during the time of the crime in court during the last trial and I would think he is sticking with it.