About the whole Momochi gets laughed at vs. Sayu laughed with, I think this video does a good job of highlighting that since Momochi and Sayu were on it together:
[youtube]rgo1jWW58rg[/youtube]
And often times with Sayu she invites the laughter. She said that she finally got a syllable in their most recent single (this was the 123 time period) and that hitting it correctly was a matter of luck.
Notice that Sayu conveniently focused on the "RU" she got in "AI SHI TE RU" instead of the fact that she got pretty prominent "Hey Baby"'s during the verses.