An ability that triggers when something “attacks and isn’t blocked” triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.

If you lose control of Orcish Squatters before its ability resolves, you won’t gain control of the targeted land at all.

The ability triggers on the declaration of blockers if no blockers are declared for it.