Aunque el vuelo tuvo una escala larga, nos gustó tanto el destino que volveríamos mañana mismo.

Not exactly.

Aunque means although / even though, and it introduces a subordinate clause:

• Aunque el vuelo tuvo una escala larga... = Although the flight had a long layover...

Pero means but and simply links two main ideas:

• El vuelo tuvo una escala larga, pero nos gustó tanto el destino...

So both can be similar in meaning, but aunque is better when one idea is presented as a contrastive background to the main point.

This is the classic preterite vs imperfect question.

• tuvo = preterite of tener
• tenía = imperfect of tener

Here, tuvo presents the layover as a completed fact within the trip:

• el vuelo tuvo una escala larga = the flight had a long layover

If you said tenía, it would sound more descriptive, as if you were setting the scene:

• Aunque el vuelo tenía una escala larga...

That version is also possible in many contexts, but tuvo fits well if the speaker is talking about one specific completed flight experience.

In travel language, una escala means a stopover / layover, usually when a flight is not direct.

So:

• un vuelo con escala = a flight with a stop
• una escala larga = a long layover

In Spain, this is a very common word for air travel.

Because in Spanish, adjectives often come after the noun by default.

So:

• una escala larga = a long layover
• un destino bonito = a beautiful destination

In English, adjectives usually come before the noun, but Spanish normally places them after. Putting the adjective first is sometimes possible, but it usually adds a more literary or emotional nuance and would not be the neutral choice here.

Because gustar works differently from English to like.

In Spanish, gustar literally works more like:

• the destination pleased us

So:

• nos = to us
• gustó = pleased
• el destino = the destination

That is why:

• Nos gustó el destino = We liked the destination

You do not usually say gustamos el destino to mean we liked the destination.
Gustamos would mean we are pleasing / attractive, which is a different idea.

Because the subject of gustó is el destino, which is singular.

Structure:

• nos = indirect object
• gustó = singular verb
• el destino = singular subject

Compare:

• Nos gustó el destino. = We liked the destination.
• Nos gustaron los destinos. = We liked the destinations.

The verb agrees with the thing being liked, not with the people doing the liking.

Because tanto is used with verbs and nouns, while tan is used with adjectives and adverbs.

Here, tanto modifies the verb idea gustó:

• nos gustó tanto... = we liked it so much...

Compare:

• Nos gustó tanto el destino que volveríamos. = We liked the destination so much that we’d go back.
• El destino era tan bonito que volveríamos. = The destination was so beautiful that we’d go back.

So:

• tanto
  • verb/noun
• tan
  • adjective/adverb

It means so much / so ... that.

In this sentence:

• nos gustó tanto el destino que volveríamos mañana mismo

This means the speaker is expressing a consequence:

• we liked the destination so much that we would go back immediately

It is a very common Spanish pattern:

• Habla tanto que me cansa. = He talks so much that he tires me.
• Era tan caro que no lo compré. = It was so expensive that I didn’t buy it.

Notice again:

• tanto ... que with verbs/nouns
• tan ... que with adjectives/adverbs

Volveríamos is the conditional tense: we would return / we’d go back.

Here it expresses a strong hypothetical idea:

• we liked it so much that we’d go back tomorrow

It does not necessarily mean a real plan has been made. It sounds like:

• We loved it so much, we’d go back tomorrow if we could.

If you said volveremos mañana mismo, that would sound much more definite:

• we will go back tomorrow

So the conditional gives the sentence a more emotional, not-literal, reaction.

Mañana mismo means something like:

• tomorrow itself
• as soon as tomorrow
• tomorrow, no less

Here mismo adds emphasis. It makes the idea sound immediate and strong.

Compare:

• volveríamos mañana = we would go back tomorrow
• volveríamos mañana mismo = we would go back as soon as tomorrow / tomorrow without hesitation

This use of mismo is very common in Spanish:

• Ahora mismo = right now
• Hoy mismo = today itself / as soon as today
• Esta noche mismo is less standard; usually esta misma noche = this very night

Yes, aunque can be followed by either indicative or subjunctive, depending on meaning.

Here we have:

• Aunque el vuelo tuvo una escala larga...

The indicative is used because the long layover is presented as a real, known fact.

Very roughly:

• aunque + indicative = although / even though, when the speaker treats it as true
• aunque + subjunctive = even if / although, when it is uncertain, hypothetical, or not the main point as a fact

Compare:

• Aunque el vuelo tuvo una escala larga, disfrutamos el viaje.
  The layover really happened.
• Aunque el vuelo tuviera una escala larga, iríamos igual.
  Even if the flight had a long layover, we would still go.

So in your sentence, the indicative is used because the speaker is referring to an actual completed trip.

Yes, that is possible.

Spanish word order is more flexible than English. These are both grammatical:

• Nos gustó tanto el destino que volveríamos mañana mismo.
• El destino nos gustó tanto que volveríamos mañana mismo.

The original version sounds very natural and keeps the focus moving toward el destino before the result clause.
Putting el destino first gives it a bit more emphasis.

So the original is not the only possible order, but it is a very normal one.

There should definitely be an accent in volveríamos.

The correct forms are:

• gustó
• volveríamos

Why?

• gustó needs the accent to mark the stress on the last syllable.
• volveríamos needs the accent on í to show the stress pattern of the conditional ending -íamos.

Without the accent, volveriamos would be misspelled.

Accent marks in Spanish are not optional; they are part of correct spelling.

It is understood across the Spanish-speaking world, not only in Spain. It is a general Spanish expression.

That said, the whole sentence sounds very natural in Spain:

• escala is standard travel vocabulary
• volveríamos mañana mismo is a common emphatic way to express enthusiasm

So it is not uniquely from Spain, but it fits Spain Spanish perfectly well.