(1) Yes, it is lawful and shall earn reward. Any dish whether Biryani or something else whether it was favourite dish of the deceased person or not whatsoever food you would feed to the poor and needy ones with the intention of sending reward to the deceased’s soul its reward shall go to the person.
(2) No, you should feed them without following the customs of tija, daswan, chaliswan and barsi as these crept into Muslims from Hindus.
Allah knows Best!
Darul Uloom Deoband