A seminarian isn't a prisoner with no rights. If he doesn't trust that a bishop is valid, he can just leave the seminary or study house, or wherever he is. This assumes he's already asked for evidence of his own concerning the bishop, and has been refused or given little answer. If he blindly trusts his seminary director concerning a bishop, it makes me wonder about the common sense level and 'street smarts' of the seminarian.
Secondly, there's 2 issues here, which are being co-mingled. 1) the validity of the priest and 2) the validity of the bishop who consecrated the priest. It seems that in this case, there are doubts about both the priest and the bishop, therefore evidence and an explanation is doubly important.