Hi. Good questions.
1. Absolutely. Kotlin is a splendid language and why learn Java or Go if you can have it all in one language? I do all my projects, whenever possible, in Kotlin nowadays.
2. Depends on your use cases. If you start completely from scratch and usually have the option to chose, go with Ktor. Spring, on the other hand, could open more doors if you are looking to switch more torwards the backend development side of things. Ktor has the huge benefit of being much smaller and easier to learn. Spring on the other hand is a full-blown and nowadays good-ish modularised one-tool-for-everything.
3. Go with the tutorials the projects provide. They get you up and running.Then, check back here with question on details (and check the internet as you will find loads of help on there as well).
4. It highly depends on what you are trying to achieve and what your background and time is. Go is nice and yet, a complete different language. Rust is also something that gains lots of traction nowadays. But then, you already know your Kotlin. So it is very likely easier to pick up a new framework or two compared to learn a new language AND frameworks.