YW fan
there are many minhagim with sefira. some keep from begginign until LAg baomer some until Lad Ba’aomer some from Rosh chodesh until then end etc.
(as an aside “listening to music” is a later addition to the minhag as well though dancing is brought earlier so your point stands)
Similarly there are many minhagim with Lag Baomer. According to the Mechaber there is no dancing at all not by day nor night. According to the Mishna berura’s understanding of the Remah you are right about Lag baomer night. But not everyone agrees (MB also brings cholkim that haircuts are allowed at night, no reason for haircuts to be different than music)
in short all of sefira is based on minhagim, and there are different minhagim