The method of retroactive love.

Given what we know about how little use Elly had for Thelma Baird when she was alive, it would make sense to assume that she play-acted mourning a woman she was cold to because she knew that despite her feeling as if dealing with her was a burden, her children thought the world of the kindly old meddler who gave them yummy treats, praised them for a job well done and many other things that got in the way of Elly's loving plan of showing that she loved her children with all her heart by being a bitter, angry nag who could never be pleased with anything. The problem with that reasoning is that under the angry blowhard and psychotic-looking malcontent, there's a born follower ready to cravenly follow a party line.

What this means is that Mrs Baird's body didn't even have a chance to get cold before Elly started revising the past to make them best friends. Elly's fear of being known to have an unpopular opinion and her living and dying by what other people might think made it a priority to not be known as the Bad Person who didn't especially miss someone she didn't like dealing with. This took the form of her telling herself that she hung off of Thelma's every word and thought of her as an extra mother and a whole host of other crap. It's too damned bad that she was too busy rewriting the past to notice what her refusal to give Lizzie a proper understanding of Death would make a mess of the future; otherwise, Liz wouldn't think that for her to enter nursing homes would always kill the person she's visiting.