Sempre/nunca don’t automatically indicate that you should use the verb ser, which is inconvenient, because it means there is no easy way of always getting this right!
Note that the whole temporary vs. permanent condition is just a general rule of thumb, not a hard line between the two verbs. For example, in sentence #1, the verb ser is used to say that Inês and Márcio are friends, and yet, we don’t know if they’ll be friends forever. Also, we preferably use the verb estar to talk about moods, which is why I wrote “estão sempre bem dispostos”. However, I could phrase it differently and write “são pessoas sempre bem dispostas”, because with the noun pessoas (people), the verb ser almost always fits better.
In sentence #2, the temporary vs. permanent thing can be followed more or less closely. The classroom was built with a fixed (permanent) size, hence “a sala de aula é grande”. But windows are usually made to be opened and closed. So, even if they are always in the same state, the expected potential for change makes us prefer to say “janelas… estão sempre fechadas”. Same for their cleanliness (“nunca estão sujas”) or for how busy the bar usually is.
One additional way of distinguishing between ser and estar is to think about intrinsic/non-intrinsic and standard/non-standard qualities of something. For example, we say “a relva é verde” (grass is green) because color is generally seen as an intrinsic attribute of something and the green color is seen as the standard for grass. When green leaves turn yellow and brown, we often use estar to describe those new colors, because those are not the standard tones. I hope I was able to explain this in a way that made sense