1) The Zomemim witnesses are liable because of what they tried to make someone lose. That is, the rule is that Edim Zomemim are liable for what their testimony would have, in reality, made their victim lose, and not merely for what they wanted to make him lose. If the husband would have paid anyway, this means the witnesses did not make him lose anything.

2) This is what the Mishnah states at the end: "If she dies, the husband will inherit her," and he will not pay the Kesuvah. Rashi (DH Omdin) explains that if this happens, "he will lose the money he gave," meaning that the person who bought the Kesuvah from the wife will lose the money that he paid for her rights in the Kesuvah; if the wife dies first and the husband inherits her, the buyer of the Kesuvah will not receive a cent.

However, the answer to your second question is that because we do not know at the moment whether or not the husband will eventually pay the Kesuvah or not, it follows that the way one determines how much the witnesses have to pay for their false testimony is to estimate what the selling price of the Kesuvah would be now. How much would the Kesuvah fetch in the market place, where a potential buyer would decide how much it is worthwhile for him to pay the woman for her rights in the Kesuvah on the chance that the woman might actually collect her Kesuvah? Since the husband could have been the one to buy his wife's rights to the Kesuvah, the difference between the actual sum of the Kesuvah and the lower amount that he would have paid for the chance not to have to pay her is the sum of money that the witnesses were trying to make the husband lose.

Kol Tuv,

Dovid Bloom