Unfortunately, even if the two things are true beyond a doubt, they do not constitute proof of any crime. They certainly are reason to investigate further, but the rules of evidence are pretty strict.
Welch may have been at Wheaton Plaza that day, but so were thousands of other people. That circumstance alone is not sufficient to establish any guilt. Now, couple that alleged admission with someone positively sighting him there AND possibly approaching the girls, talking to them, etc, and you build your case. BUT still you would have to prove that he abducted the girls, and or killed them. And there is just no forensic evidence of that yet.
Welch being a sex offender is another factor which makes him a viable suspect. But, again, it is not proof. In a trial, you cannot present misconduct not specifically charged. In fact, even when someone is convicted of an offense and the trial moves to sentencing phase, a defense lawyer would argue against the jury hearing of any sex offenses which took place AFTER the crime he is convicted of, because it is prejudicial. Theoretically, a person might kill someone and LATER become a sex offender. The two crimes (it would be argued) are mutually exclusive.
This is in fact what happened with Fred Coffey. He was first convicted of sexual misconduct with children (occurred in 1986). Next, he was convicted of the 1979 murder of a child. In sentencing, the jury heard all about the sex offense convictions and took those into consideration when they voted to give Coffey the death penalty. Coffey was granted a retrial which excluded mention of the sex offenses, and the new jury sentenced him to Life in Prison.
That said, it is indeed significant that this Welch has an evil background and that he might be proven to have been at Wheaton Plaza that day in 1975.
I believe that there are some other potential suspects who are more likely than Welch. Unfortunately police have not proven that any of them were actually at Wheaton Plaza that day.
