I think every character is cliche to an extent. I mean, people have been writing stuff for centuries, and anime itself is pretty iterative. I think "cliche-ness" is only a problem when its used as a crutch or easy characterization without spending time, something like "oh, you know what a tsundere is, we'll just give this girl blonde twintails so they know." But if you think about it, a tsundere is literally a person that starts off cold or uninterested and slowly warms up to people... and doesn't that describe most people? Steins;Gate's characters are kind of cliche, but they're suitably fleshed out in my opinion. None of them are their cliches just for the sake of being cliches (Kurisu is very understandably tsundere given her past and Okabe's weirdness, Ruka goes beyond the idea of an otokonoko and is probably just actually trans, etc.).

I don't believe so. I'm pretty firmly on the side of authorial intent not having any particular authority over any other interpretation, but rather it is in the pool of interpretations one may draw from. For example, let's resort to a common argument from called reductio ad absurdum. Let's say J.K. Rowling can't take it anymore, and comes out saying that Harry Potter was actually a piece on how courage and friendship are illusory and will only bring you unhappiness in the end. This is pretty much the antithesis of what Harry Potter actually says, But hey, authorial intent right?

While something this extreme would probably never happen, the point stands—I think each work should be judged on its merits and what's in it rather than looking to outside sources. Not to say that outside sources can't help nudge one towards a particular interpretation, but in my view the work is king, and side-steps the issue of something like poor writing (can an individual intend something but completely fail at executing that intent?).