Yes, it would be perfectly possible for an elf to marry a mortal and remain immortal, and the default case, just like Saucepan said. This was the crux of all elf/man marriages (all two of them). The other side of it, where it doesn't end well in some way would be the case of Andreth the Wisewoman and Aegnor in the First Age. Both were in love with each other, but both feared to make the move (cf. HoME X, 4).
A family tree of Dol Amroth is given in History of Middle-earth XII, 1, vii. The original ancestors of the house were Imrazôr the Númenorean and Mithrellas, a companion of Nimrodel. The elf/mortal marriage and half-elven question as regarding this case is not addressed there, however. Presumably, there was nothing special about this marriage because Mithrellas was of the 'lesser Silvan race', not of the Eldar. I seem to remember this was more or less discussed before, not necessarily satisfyingly.
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