Learn Spanish (Spain): ¡Ojalá este país fuera un paraíso sin estrés! - If only this country were a paradise without stress!

Because the speaker feels this wish is unlikely or impossible right now. When we use ojalá with the imperfect subjunctive (like fuera), it translates to 'if only...' and expresses a longing for something contrary to the current reality. If they thought it was a realistic possibility, they would use the present subjunctive (sea).

Fuera is the imperfect subjunctive of both ser and ir. How do we know which one it is, and why is it used here instead of estuviera?

Here, fuera comes from ser (to be). We know this because being a paradise is a defining characteristic or identity, not a temporary state or location, which would require estar (and its form estuviera). It cannot be ir (to go) because the sentence would make no sense.

Is the word que required immediately after ojalá?

No, it is completely optional. You can say Ojalá que este país fuera... or just Ojalá este país fuera... with absolutely no change in meaning. Dropping the que is very common in everyday Spanish.

Shouldn't este have an accent mark (éste)?

No. Este is acting as an adjective here because it comes directly before the noun it modifies (país). You only used to put an accent on it when it was a pronoun replacing the noun (meaning 'this one'), but even that rule was dropped by the Spanish Royal Academy years ago.