Yes, she struggled, but it wasn't to the point of her being less than 1/6th of his strength.
That's true, but she could've used her magic to move the moon and the pendant to move the sun. She clearly had problems controlling the pendant, so it'd probably make her job easier if she only needed to use the pendant for one part.
When did he try to move the moon and fail? Also you could call him not being able to move the sun PIS. it was necessary for the continuity, since in season 2, it was stated that unicorns (plural) moved the sun, and the writers didn't want to retcon it, so they had to say that Starswirl just provided the majority of the energy.
I think you worded that wrong. You basically said he can lift the sun by himself. But if that's not what you meant, then...Um...yes, he does. Shown as comparable to the princesses in the premier, being the princesses' teacher, being so hyped up over so many seasons to be like the strongest pony ever, being able to shield against the PoS attack, being stronger than someone who can fight Sombra, and also, if he, and thus Twilight are low 4-C, then other things don't make sense. For example, Tirek and Chrysalis equals Third Form Tirek, who was confident in taking on three alicorns, so them both being Low 4-C just doesn't make sense. Especially if it's like less than halfway through the tier. That would mean them combined shouldn't even be 4-C, which obviously isn't the case. Also, he was he leader of a group that protected Equestria, which almost always has 4-C threats. I know this isn't concrete proof, but it is supporting evidence.