If the creature that triggered Portcullis is no longer on the battlefield when the ability resolves, then it fails to do anything.

You check the count of the number of creatures on the battlefield again during resolution. The creature will not be exiled if the count is 2 or less.

Creatures which are phasing in will not trigger this card's ability.

The "when Portcullis leaves the battlefield" ability is set up as part of the ability that exiles the creature. It will trigger when Portcullis leaves the battlefield even if Portcullis has somehow lost its abilities at that time.

The creature does enter, so any other abilities that trigger on it entering trigger. This follows the normal rules for timing triggered abilities: first, the current player puts their abilities on the stack in any order, then each other player in turn order does the same. Portcullis' own ability is included in this, so it could result in the creature being exiled after some abilities have resolved, but before others.