Im having trouble believing what Fate fans are saying in this event, cause so far most of what they claimed ended up being false when reading the wikia or an impossible stretch of their abilities BUT I will check the wikia again (for the upteenth time) to see if its true later, will take your claims at face value now.Well if Day 1 was me convincing you he can be damaged then that's a start. A wise man once said "if it bleeds it can be killed", so don't be so gung-ho with assuming your boy's victory is guaranteed.
As you read, Musashi's Heavenly Demonic Thundering Eye allows her to see through her opponent's defensive actions, and demolish them. This skill is legitimate because in the FGO game she pierces through invincibility with it, damaging her opponents as she wishes as well as the ability reducing her opponent's defences. You said that this skill convinced you she might be able to damage Reinhard, well I say given what it does it's a guarantee she can damage him and by extension, kill him.
A battle is only ever settled when someone loses or they forfeit and I'm not about to do either so settle down Reinhard has these magical blessings for sure, but have we discussed the possibility of Musashi not severing them from him but sealing them? I am referring to her 2nd noble phantasm, Ganryū-jima: I Was Told There Would Be Magic Sword-Busting! This Noble Phantasm is one she uses against any sword-wielding opponent who doesn't just rely on their skill with the blade, with this Noble Phantasm breaking their blade and sealing their skills. This is yet another asset she has that can turn the tide of the battle, and as someone who fights up close, if Reinhard suddenly loses his blade to defend himself then the Heavenly Demonic Thundering Eye not only guarantees Musashi can hurt him but gives her an optimal chance to kill him. I still contest that she can incorporate the Swirl of the Root to damage his connection to Od Lugana and switch off Reinhard's blessings, so a sword attack using the swirl of the root should be the end of Reinhard.
She is a far better fighter than you give her credit for...
The reason I said Musashi would be able to damage him is because the technique you talked about ignores physical defenses, and Reinhard was damaged by an opponent that reduces any kind of defence to absolute zero (Regulus).
He is capable of resisting a thing like that tho, thanks to the Blessing of Spirits that makes him instantly heal fatal injuries before they kill him (what Julius has but on steroids, he cannot speak with the spirits like Julius does tho because there are too many and their wills are only centered on protecting him).
Thanks to this he was capable of grabbing Regulus for short periods of time or punch/kick him without vaporizing himself in the act, but he still took damage from doing it... Which made him bleed... which made him stronger...
The second NP you talk about wouldn't work either since his swordsmanship is at the absolute peak without blessings, on pair with Reid, who could do that Musashi's attack that ignores any defence with chopsticks (Reinhard can replicate it with the Dragon Sword).
Yet on top of that he has the "Blessing of the Sword Saint" which maximizes the user's martial prowess to their maximum possible limit (see above), allows them mastery of any weapon as long as it can be wielded (a stick, a whip, a spoon...) and allows them to draw the Dragon Sword from its sheath but only if the sword finds the opponent worthy.
Finally if she's capable of cutting his connection to Od Laguna it will instantly repair itself. Musashi would need to eliminate Od Laguna for the connection to be severed completely, which is the same as destroying the Universe.
On top of that, excuse me if I highly doubt she can do that, since if she could all she would do to defeat any servant is severe connections to the Root and have them disappear, which would make her NP... EX+ maybe?
But hey, Im taking your claims at face value, still she cant do it. I agree she's strong btw, would defeat a lot of nominees imo but Rein is just too much.