Is this in the EBT or cultural? Does sharing merit ,work?
Yes, see Kp7. Seemingly, this only refers to hungry ghosts and not other sentient beings.
Just as the rivers full Yathā vārivahā pūrā, swell the ocean seas paripūrenti sāgaraṁ; so too what is given here Evamevaṁ ito dinnaṁ, aids the departed ghosts. petānaṁ upakappati.
Thinking: “They gave to me, they did for me, Adāsi me akāsi me, they were my family, friend, companion”, ñāti mittā sakhā ca me; give offerings to departed kin, Petānaṁ dakkhiṇaṁ dajjā, remembering past deeds. pubbe katamanussaraṁ.
I wonder, with so few sutta references to this practice — even if only for the afflicted spirit realm — to what extent may it be a possible late-addition to the Canon?
The idea of ‘sharing merit’ seems less a late addition to the Canon and more of a traditional Theravadin tradition evolving after the Canon was closed.
Though I’ve no idea whether the practice benefits pretas in any way, I do believe it “works” in the sense of being beneficial to those who do it.
Firstly, within a funeral context, the belief that they can assist their departed loved one serves to assuage the grief of mourners. Secondly, in its more general undertaking, merit-sharing essentially amounts to a modified form of mettā- and karunā-bhāvanā.
Not an EBT source but the biography of Mae Chee Kaew has an interesting account of what merit is and how it can aid a departed being.