For the record.
The ponevezher rav flew the flag because of shlomo shel malhut.
much like he would fly the union jack in britain and the stars and stripes in the usa.
There is no record at least as far as I know about not saying tahanun on Yom Haatsmaut.
In his yeshiva they do say tahanun on Yom haatsmaut. They said when the rav was alive and afterwards when his son R avraham was the nasi of the yeshiva, and they still do now.
In both ‘branches’.
In r tzv markovits’ yeshiva and in r eliezer kahanemans yeshiva.