As happy as since (please different opinion would be appreciated)

It would be nice to get different opinions because not everyone has the same version.
I've been getting different answers. I would be nice to get the same answer. What do you think?

Which is correct?
She's never been happier than since she met him.
She's never been happier since she met him.
Since she's met him, she's never been happier.

Which is correct?
I've never felt better than since I've been on a diet.
I've never felt better since I've been on a diet.

Should I leave 'this' or not?
They've never been [this] happy since they moved in together.

Without 'this' = They have never been happy since they moved in together in 2008
With 'this' = They have never been happier than since they moved in together in 2008. They are happier than they were when they moved in.

Which is correct and what does it mean? It seem to have 2 meanings.

Since they moved in together, they've been the happiest they've ever been.

Meaning 1
Since 2008 (date of move) they've been happy and it hasn't changed
Meaning 2
Since 2008 (date of move) they weren't happy until the comment was made in 2010.

Does this mean anything?

Since they've moved in together, they've been happier than they've ever been before.

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Are these both correct?

He’s never been as calm as since he started preschool.

He’s as affectionate as when he we started dating.

Thank you for your time and answer