If a Kalla who is not receives a kesuba written for a besula, is her marriage kosher?
Yes, provided her husband knew about it beforehand.
And if so, is her kesuba kosher?
Again, yes, provided that the husband knew the true situation beforehand.
What if she never informed him of her status, and he only found out after?
Then you may have a case of a mekach ta’us and a rav should be consulted. If the husband has no post facto complaints there is no reason the marriage cannot continue* and there is a high degree of probability that the kesubah would still be valid anyway.
The Wolf
* Barring kohen – zonah/g’rusha issues.