A way to communicate between each other that isn't shouting is something I'd recommend, since any Servant of decent mobility (and the castle) will be breaking the sound barrier frequently. If anyone has magecraft or abilities suited to that, it'd be of great help.
The easy way that most mages there could pull off would be using Reinforcement on their vocal cords, but that still counts a shouting, so yeah, not very efficient.
tfw Airi feels useless but she's actually the only one there who has a decent way of communicating with their Servant, lol.
Whether or not Masters and Servants can inherently communicate telepathically tends to be inconsistent between various Fate works. In Fate RP's, I usually assume they cannot, unless it's specifically stated that they can.
As for magics possessed by OC's that could specifically help in this situation, would Xavier's displacement magecraft be able to work on sound waves? That way Masters could talk to their Servants without shouting and without wasting Command Spells. It sounds kind of iffy though, so what do you think @Moonlit Sonata?
I have no objection to letting my partners control my characters. Moses won't be very useful in this fight (unless you want him to punch the thing) and Marguerite will stand off to the side and let Sumanai do his thing. Which is hitting things.
I could handle Moses during your absence. I don't know anything about his abilities other than what's written on his character sheet though. From what I see, both his NP and his wrestling moves would be all but useless against the golem. Does he have any support magics? What about his Charisma? Would it boost the party or nah?