Learn French: Est-ce que l'individualisme est un problème pour l'éthique ?

In French, abstract nouns and general concepts always require a definite article (le, la, les, or l' before a vowel). While English drops the article for general ideas, French strictly keeps it.

Could I use inversion here instead of est-ce que to sound more formal?

Yes, but because the subject is a noun (l'individualisme), you have to use a structure called complex inversion. You state the noun first, then add the corresponding inverted pronoun: L'individualisme est-il un problème pour l'éthique ? Using est-ce que avoids this complication and is very common.

Why is l'éthique singular when the English word "ethics" ends in an "s"?

In French, l'éthique is a singular feminine noun when it refers to the general philosophical concept of morality. Even though the English equivalent looks plural, it is treated as a singular, unified concept in French.

This lesson is about the pronoun soi (oneself). How could I use soi to talk about l'individualisme?

You could describe individualism as le fait de ne penser qu'à soi (the fact of thinking only of oneself). Because individualism is a broad concept that applies to people in a general, unspecified way, the indefinite pronoun soi is the perfect choice for defining it!