2. (a) That's right. An action for trespass to land is available to a person in possession of land, even if they do not own it. This includes a tenant. A tenant is even entitled to sue their landlord (the owner) if the landlord trespasses on the leased property.
The onus rests on the alleged trespasser to prove that they did not voluntarily intrude onto the possessor's land, either intentionally or negligently. Mistake is not a defence.