That it is apparent Imrahil has elvish blood, just means that his bloodlines are more strongly Numenorien than most. Which means somewhere in his family tree he is a descendant of Elros. Although it may be possible that he had both Noldor and Teleri ancestry.
As far as the title of Prince, his bloodlines may have made him a possible claimant to the throne of Gondor. (I'm at work and don't have anything handy to reference, in order to summarize why Gondor remained under the rule of the steward's, but JRRT explains this in the appendixes of ROTK I believe). Although it may be a title similiar in rank to that of a Duke, whereas "Prince" means of royal blood, opposed to a lesser noble bloodline.
Or, given that Dol Amroth is a principality of Gondor, Prince may be the titular denomination given to the heads of Gondorian principalities, whom more than likely had royal blood, or a type of lesser King. Which opens up the possibility for more than one Gondorian "Prince."
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