Hi Dominik! The second one it's like "cause and effect". When you say "Paco y María estaban limpiando la cocina", the person who is listening to you wait for the second part of the sentence. Estaban limpiando + complement. They were cleaning + when a mouse appeared.

Both sentences are correct and you can use them to say something in past.

Yes both are correct

Both sentences are correct BUT in different time forms: "Limpiaban" is in 3th person prural of imperfect "pretérito" "Limpiando" is in "Gerundio", normaly with the auxiliary verb "Estar", in this case "Estaban" (prural "Estar" form) Both mean exactly the same

both are correct