@Remathilis @Whizbang Dustyboots @Azzy
I'm not going to respond to each of your posts, instead I'm going to show you how I would have done it if I were at WotC and I have posted this elsewhere so you may have already read it.
The way I see it with 5e, since they want to go the route of the original world being shattered and aspects appeared of the dragon goddess within the multiverse, is that...
There indeed was 1 Chromatic Dragon Goddess (name perhaps lost in time). Her universe shattered and aspects of her splintered into the multiverse. The Forgotten Realm's aspect took on the name Tiamat. The DL aspect took on the name Takhisis. They evolved within their respective universes. Although their heritage is the same, they are indeed different beings. They both have some common traits but they are for all intents and purposes different creatures.
Think Michael Keaton from Multiplicity. The original Michael Keaton is destroyed, only the copies now remain but they are indeed different beings. WotC definitely dropped the ball on this IMO. WotC going Tiamat = Takhisis is about as creative (and respectful) as David Benioff's and D.B. Weiss's storyline for Dorne, which is to say not at all.
EDIT: Like it or not many if not most of the DL fans came to the setting via the novels. When I'm talking about creators of the setting - for me it would be the authors. I read way more novels than I did DL gaming product. We can agree to disagree if it was lazy or whatever. But every edition adds new lore or amends old lore. They could have fixed the 2e Tiamat = Takhisis. They decided not to and I'm calling that lazy.