Why should Leira be dead in your games, and why would you have to wait and hope for the possibility that Leira might return in 5th edition Forgotten Realms? I too, always liked Leira as a goddess in the Forgotten Realms. It's your choice in your games whether or not you follow the events of some really horribly written books, puposed to explain changes in a game system.
I've read several of the FR novels, and there are few that I've liked. Those I have liked still do not rank very high amongst so many fantasy books and series from much better authors with much better writing skills, and who were not leashed by game system dogma. I've used a couple of homebrew settings for most of my D&D gaming, but as far as published settings go, FR was my first, and one of my favorites that my group and I return to from time to time. So, even though I've played post 2eAD&D "Time of Troubles", 3e and 4e, I've never stopped using the AD&D greybox Forgotten Realms Campaign setting.
In our FR based games, Leira is still the Lady of the Mists. Bane, Myrkul, and Bhaal still are the gods of tyranny, death, and murder. Elminster is just the sage of Shadowdale. Not the god that Greenwood has written Elminster into being just because Elminster is his character in his setting and so that no player character can ever match him. And Drizzt is just some drow that surfaced in the north that players from that region may, or may not have heard of. And not the iconic "every drow character made, ever" that all the Drizzt fanbois hopping the Drizzt bandwagon have made him to be.
We all have our own opinions of the FR novels and their authors. We all have our favorite version of the FR setting. Just use the one that you and your group enjoys the most. If you like Leira, then use her, whether or not you use the old greybox FR setting or whichever FR setting is the newest and shiniest. If you like the greybox, but wish to purchase newer FR setting versions for the system statblocks and easier, faster conversion, that's cool. It's an RPG. You don't have to have everything written down as law or follow the novels and newer edition versions as canon. Use what you and your group likes best.