Roel van M.
My interpretation is that there is only one person in first place on the wall - the person on top of the front-most stack. If you're in the front-most stack but not on top, you aren't in first place (and therefore may advance using Sentry). I wouldn't base my decision too much on similar wordings on other cards - cards with the same intent in this game have been translated to have slightly different wordings, and in general the English translations are pretty poor.
If you use the sentry, you can still pay 1 AC (not GC) to advance 1 space, 3 AC for 2 spaces, 5 AC for 3, and so on, regardless of whether or not you've already used Sentry.
These aren't official responses, but this is how I would rule in these situation.
That's what I think too.