Chainrings are made with sculpted teeth nowadays to aid shifting, so they tend to look "worn" even when brand new. You'll see some rounded off teeth and teeth that are shorter than others. A truly worn chainring generally has teeth with a consistent "shark fin" profile all the way around the chainring. And since the load from the chain is spread out on many more teeth, chainrings tend to wear much more slowly than cassettes. If you've been replacing chains regularly, chainrings will typically last through a lot more than three chains.

Yes, you can, but you may find that shifting performance isn't nearly as crisp. And trying to get your front derailleur adjusted juuuuust right may become a futile exercise.

You might find that straying too far from the stock chainring sizes degrades shifting a bit, too, since modern derailleurs are sculpted with specific chainring combinations in mind to aid indexed shifting systems.