1) Sí, derjatsela is wrong, it means "Yes, to leave it to him"
You have to say "Se la dejamos"
In old Spanish (like centries ago), sometimes an unconjugated verb could be a command. Like "Miraros" (Look at yourself), but it is not correct now adays and besides, you are not saying a command, rather a statement in the past tense.
2) Here, you need to say "Peuede tarernosla" OR "Nos la puede traer"
The direct and indirect objects NEVER are between the auxiliary verb and the second verb. If you were studying French, it would be right to say "Pouvez-vous nous l'apporter" but in Spanish the object pronouns either go all before the auxiliaryt verb (in this case, poder) or attached to the second verb at the end
Trilingual household (French, Spainish and English)