meat for seudas rosh chodesh av

The Minchas Eluzar brings down (Sha'arei Yisaschar os 9 on Av) that the practice he saw among tzadikim was that they would eat meat on Rosh Chodesh Av as seudas Rosh Chodesh even without a siyum. I have wondered about this in the past myself - why should Rosh Chodesh itself not push off aveilus for an extra day - but the simple reading of Shulchan Aruch is that the practice of aveilus starts from Rosh Chodesh, meaning on that day itself. The Bnei Yisaschar (Tamuz-Av, Mahus haChodashim, 10) goes so far as to suggest that the leftover meat from seudas Rosh Chodesh or Shabbos may be consumed during the nine days! The proof for this astounding chiddush comes from the Rosh. The gemara in Chulin writes that until the conclusion of the conquest of Eretz Yisrael, 'basar nechira', meat which was killed without shechita, was permitted. The gemara leaves as an unresolved question whether basar nechira which could have been eaten before the end of the period of conquest was still permitted afterwards when the mitzva of shechita applied - do we say that the meat takes on a status of heter which is not lifted afterwards. The Rosh takes this historical question and derives an application l'ma'aseh - if one took a neder not to eat meat, would the leftovers from a previous meal be permitted and only new meat be assur, or would any meat now be prohibited? Since not eating meat during the 9 days is only a minhag, some suggest being lenient on the safeik of the Rosh - once the meat was permitted for seudas Shabbos or Rosh Chodesh, the leftovers remain with the status of heter. The Bnei Yisaschar is unconvinced of this ra'aya, and the Aruch haShulchan (551:24) rejects it for his own reasons. Aside from their reasons it seems that one can make a lomdish chiluk, but I'll leave that for thought (or the comment section).