Those would be logical problems. However as shown with Derek, blizzard doesn't care about that kind of stuff. But sure, assume they don't care about the ships or that they are "impossible" to find(lmao), Slyvanus should still be interested in Dreven's corpse. She is hinted at having the power to raise elves, why wouldn't she try to find him and revive him? Because the San'layn are failures and she doesn't care about them anymore?
Why do you assume most San'layn would care about free willed undead? The San'layn seemed fine serving the previous Lich King.
They are very unlikely simply due to the fact they are elves which there is already plently of. A belf blood DK is already a San'layn anyway.