Mallory:
DM - is the figure that Astre has found on the shore nearest the party, or opposite across the ice floe?
Nearest the party.
I edited the existing map to indicate the path you've followed.
Mallory:
Also, is he near enough for telepathy/sense sharing so that Mallory can relay his findings, or is the narrative of what he's seeing just for him alone?
You're not pressed for time, so either is fine. Assuming he's directly overhead, 100' would amount to quite a bit of vision from where you are, seeing as the lake is completely flat. Or we could say he's farther up ahead and can loop back whenever he deems fit.