Barnabus:
Is it apparent by which means, if any, V. could achieve long-distance ingress into this chamber? That is, would we think he's a round or two behind us?
No, the party wouldn’t anticipate being followed. The means of shifting between this location and the Barrowmaze clearly stems from some magical/clerical working. As noted elsewhere, Varghoulis is a warrior (with some spell like powers). He doesn’t possess the means to activate it. Mild spoiler: the portal(s) he utilizes are “keyed” to him and his soldiers.
Barnabus:
Is that a summoning circle? Is it apparent by which means we might ruin it?
Potentially. It is definitely a magical circle of some kind. Uprooting some of the stones which comprise it and disrupting the pattern would effectively destroy it.
Barnabus:
Let's pretend Ishmael's still with us. How deep into Moria does he reckon we are?
Ishmael would note that all of the available descriptions of the ancient temple north of Helix described it as a
stone edifice. This is a wood walled room with a dirt floor. The party’s former burglar would estimate that the company was one level below ground. As in, the stairs seen in the room likely lead to the surface.