@ARSo
1] We cannot fully appreciate what it means to live under a medieval absolute monarchy. With a medieval populace.
We also cannot fully grasp the ahrayut of being at the helm of a vulnerable Jewish population, not only vulnerable in physical manner at the whim of the above, but also vulnerable in a spiritual sense where huge chunks of this population were torn of it, converting to the overwhelmingly dominant religion of Xtianity .
It was under these circumstances that RAMBAN struggled to provide physical protection to his flock [losing a high profile disputation like this one , could result in exile deportation or worse] and spiritual protection for them , as they were constantly bombarded by zealous preachers whose threats, blandishments and bribes they were forced to listen to.
It goes without saying that RAMBAN fully realized the ultimate stakes of this debate and the impact it would have. And prepared accordingly.
This wasn’t just a flippant conversation over coffee and cake.
He for sure took into account any possible rejoinder from his opponent[s] and its possible repercussions.
Nevertheless he stated , and publicly disseminated written records thereof , that a mashiach who died or disappeared without fulfilling the nevu’ot is unequivocally a mashiach sheker , exhorting all of Spanish Jewry TO BASE THEIR BELIEF on this.
Did he not expect or anticipate a challenge [by believing Jews or lehavdil learned apostates] from sanhedrin about Daniel , according to one pshat in Rashi ? What would he say if he would be challenged ?
Saying : “I disagree with Rashi ? BASE YOUR BELIEF on my interpretation”, if there exist valid alternatives ?
The fanatical debating clergy would seize this like a hungry wolf would a fresh carcass.
All this per force went through his mind. And he nevertheless stated that a mashiach who died or disappeared without fulfilling the nevu’ot is unequivocally a mashiach sheker , exhorting all of Spanish Jewry TO BASE THEIR BELIEF on this.
It is obvious , therefore, that RAMBAN considered the gmara of sanhedrin as a non challenge.
For the simple and logical reason I mentioned – Daniel never laid claim to Messiahship and did not start it. Therefore he did not fail and is unsullied .
As opposed to j who clearly started and claimed messiahship and failed .
This distinction is so ironclad and obvious that RAMBAN was prepared to BASE Spanish Jewry’s TOTAL BELIEF on it.
2] Re Ramban arguing on RASHI whenever he disagreed with him. I am unable give an ultimate absolute positive answer. But , go through his peirushim al shas and al hatorah , on halacha AND ON AGADETA , there are countless places where he argues and raises objections with RASHI explanations.
Why would he stay silent on this one ? A quite important one, if you ask me.
Again , this is not an argument for example, about makat dever whether Egyptian animals in their houses were subject to the maka or not ? Which happens to be an argument between RAMBAN and RASHI.
Remember ,we are talking here about something which people have to use as yesodot for their ikarei emuna.
RAMBAN is not arguing with RASHI on this. There is no reason to assume so, aderaba , there is ample reason to take on the other way, as mentioned.