What a dream team you three are! I"m overwhelmed by the information you have provided. The answer to "Who was Lt Mylrea of the Harpooner?" comes down to whether Daniel s/o Thomas Mylrea & Rose Savage married Mary Hughes or whether that was another Daniel Mylrea. The Manx members of the team will have to answer that one.
If he did marry Mary Hughes, then he was having 7 children between 1789 & 1805, only one with a suitable name for the Mylrea kids on the Harpooner (ie. Mary b 1796). So this Daniel was not on the Harpooner. Also, he did not have time to get married again and produce Fred, Sophia & Rose. Fred, for example,was born in abot 1803. Mary could have been from an earlier marriage if Mary Hughes had died (say) in the late 1790s - but she was having babies instead.
If Daniel s/o Thomas Mylrea & Rose Savage did not marry Mary H, then we are in better shape for thinking that Lt Daniel Mylrea was the son of Thomas & Rose.
Working backwards from my information, Fred [who survied the wreck] was born in 1803 in London. He was the son of Daniel Mylrea and Sophia Bozeley. His father was an army officer. That comes from Fred's death certificate.
All the information we have gathered keeps pointing to Thomas & Rose as the parents - EXCEPT Mary Hughes.
Over to you all, and thank you again for all this wonderful information & research. Happy to take this off-line if it's taking up too much space.