Interesting, never knew that...
See Hayom yom for gimmel sivan.
AFAIK, this has nothing to do with sefirah, it's rather a special inyan al pee (no pun intended) kabalah to cut the hair on erev shvuos.
CMIIW.
Could be, just says that the Rebbe Rashab was displeased by those who did it in the first 2 days, doesn't say why.