“You cannot celebrate during the sefira.”
I have actually never heard that given as a reason for not celebrating Yom HaAtzmaut. Do you know anyone who says that?
Even though I do not believe in the celebration of Yom HaAtzmaut, I would agree with Avi that the fact that it’s in Nissan is not a reason not to celebrate it (unless you have sources stating that this is a reason).
My argument with Avi is that he referrred to the “aveilut” of Yom HaAtzmaut which sounds like he meant the live music aspect (is that what you meant, Avi? If not, your post should have been phrased more carefully, I think), and the fact that he used the term “certainly”.