I think this should be accepted: "That is not the Matěj that is getting married to Žofie."
I'd translate that ", jenž se žení se Žofií." or ", jenž si bere Žofii." (or the same with který). But sometimes it is hard to say where to draw the line.
I agree that the structure of the proposed alternative translation is not similar enough. Did something go wrong in "žení se se"? "bere?"
Yes, just one extra "se", corrected.
I think "That is not the Matej whom Zofie is marrying." should be accepted.
"That is not the Matěj whom Zofie is marrying." is accepted.
THE Matej? Definite article cannot be used with personal names
It can in cases like this one. You have to stress the "the" audibly. Think there are different Matejs and you emphasize that this is not the one yourl listeners thought of initially.