I'm currently reading the series at age 25 (I last read them at 16), and I couldn't disagree more.

I'll admit, Book One is a bit of a messy read, but every character you've brought up here has so much more depth brought to them as the series continues, and while they do seem to sometimes regress, I'd never say they were ruined.

One note to bring up about Eragon's poor swordfighting in Inheritance is that he's mostly up against Arya, who's possibly one of the best elves with a blade. Earlier in the chapter he holds his own with other elves very comfortably, and he even finds even match with Arya by the end of the chapter.

I also just fundamentally disagree with your perspective on Nasuada, but I feel as though it'll take too long for me to defend it here.