Learn Finnish: Tämä asento on huono, koska minun selkäni on kipeä. - This position is bad because my back is sore.

The noun asento (position or posture) is formed by adding the -nto suffix to the verb root seen in asettaa (to place or set). This suffix turns verbs into abstract nouns or outcomes, so the "outcome of placing yourself" becomes your posture.

asento looks a lot like asunto. How do their derivations compare?

They both use the outcome suffix -nto, but they come from different verbs. While asento comes from the root for placing (asettaa), asunto (apartment or dwelling) is derived from asua (to live). The suffix turns the action of living into the resulting place of residence.

Is it strictly necessary to include the -ni suffix in minun selkäni?

In standard written Finnish, yes. When you use a genitive pronoun like minun, the noun must take the matching possessive suffix (-ni). However, as a B2 learner, you already know that in everyday spoken Finnish, you will almost always hear just mun selkä, shortening the pronoun and dropping the suffix entirely.

Why use kipeä here instead of a word like sairas?

While sairas is the standard word for being medically sick or having a disease, kipeä is the natural choice for localized physical pain (like a sore back or a headache). kipeä can also be used for feeling generally unwell with common ailments, like catching a cold.