OJ I haven't posted before, but like many many others I'm sure, I'm just 'keeping you company' through this and sending you my very best wishes.
I wanted to say: don't give Elliot a present and say its from his granny. Don't burden yourself with the knowledge that you've lied to him. Also, as someone said he might thank her next time he sees her. Also, if you were caught out in this, I don't think she'd thank you for it. By the sound of her she wouldn't be thankful that you prevented Elliot being disappointed in her but would just interpret it as you being critical of her again. I think the best way to deal with her is to realise she does not function like the average human being with the usual quota of compassion and selflessness, to put it mildly.
How to explain the lack of a presi from gran to Elliot? Maybe say she is so busy and so worried about his dad just now she hasn't had a chance to get one yet. Could you ask Mike to remind her?
Surely she is not punishing you through Elliot? (Tho' I suppose I wouldn't put it past her)
Hope the present issue gets sorted out with the minimum of upset to all concerned.
(PS Am regular mner but have changed my name recently - never liked the old one )