In doing this quest a couple of times and talking to others who did it I think there is a trend in drop rates. Three of the items are dropped from mobs and the 4th is a nm. The two times I did it I was around 2/650 on Brooks for the head, others I talked to were a lot less (like inthe 40's-50's), but had problems on other drops (Creek mostly).
I am thinking the reason one of the drops is a nm and that drop is 100% is because you should get it LAST. As in the more mermaid items you have, the lower the drop rate on the others. Both times Brook (Head) was my last and in reading these posts and from experience its seems if your last item isn't the nm item you have a horrible time getting last item TH or no TH.
I am going to do this quest again getting the nm item last to see what my drops rates are. Since the nm is an easy pop I would highly suggest getting other three items first.
Good Luck!