Learn English – difference between compensate and recompense

Inside may be used while describing position/location as opposed to outside. Example: The box is blue colored inside and black outside.

Within may be used for time/space. Example: I'll reach there within ten minutes. Stay within the boundaries.

The biggest differences are frequency and register:

1. Inevitable is about a hundred times more common than ineluctable.
2. Inevitable is neutral or slightly formal, while ineluctable is very formal and sounds quite literary.

Prioritization is very important in language learning. Inevitable is a much more important word for most learners to know. There are native speakers who don't know what ineluctable means.

In terms of meaning, there's a lot of overlap, and I think you could often use either word without a change in meaning. But I think ineluctable is often closer in meaning to inescapable, and tends to be used figuratively for inescapable conclusions, facts we can't avoid acknowledging, and so forth.