I'd ordinarily say there should have been no issue for Nayak to marry Tara.
It indirectly clears the first hurdle ("D-T" cannot be together, as said by the baba). In their minds, they know who they are- so it really shouldn't be a problem.
It would also afford Dhruv a degree of immunity from the Royal family shouting "m-dand" whenever they see him, even if he shows his real face later. They would never execute the husband of the princess, certainly not so soon after her marriage.
But this is ITV, and we need drama to carry forward the story and keep the marriage from happening, so they've used this track.
In a way, I kinda agree with the thought behind it, because Dhruv would ideally like to clear his name and marry Tara in front of everybody as Dhruv. And I thought the makers put some reasonably compelling flashbacks to present their case (the D-T mehndi and their names together by the flute)
Still, one feels he could still marry her as Nayak and then work towards proving Dhruv as innocent tbh- the two aren't really mutually exclusive IMO.
PS- at some point in today's episode, I thought Nayak's voice was not really like his usual gruff voice, but nobody present seemed to notice 
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