The combination of tenses?

I don’t know if “were” should be interpreted as past tense. It might be a subjunctive. When you say “We have thought”, it creates a hypothetical.

@Ticce
1. I do not know how you can assume that they are not currently dating without a time marker or any other context clue that indicates closure. This sentence is in past continuous and it usually needs another action or a time marker to signify more precisely when the action finished.
2. Yes, it can be interpreted like they have been dating until now, again because #1 doesn’t have a time marker to suggest the action is not taking place any longer.

@LostInParadise
were functions as a subjunctive when it is used in the first or third person singular, e.g., If I were you, I would buy a new car. or If I were in your place, I wouldn’t buy a new car. (But I am not you: subjunctive—a hypothetical situation.)

were + ing is a past continuous verb phrase that indicates that an action was continuously taking place in the past when another action took place: e.g., You were taking the trash out when I called.

we have thought is not a hypothetical sentence (subjunctive), it is factual (indicative). They (we) are indicating that they had assumed something, possibly because of certain circumstances that would lead to the notion they were a couple.

You might be right, but “we have thought” still seems hypothetical. Saying, for example, “We have known that you were dating” does not sound quite right. It seems that it would be more appropriate to say, “We have known that you are dating.

^I understand, but what is hypothetical is if they were dating or not, not the fact that other people (we) thought they were, that in itself it’s a fact.

It was our contention that, for a while at least, they were dating.

Okay, I’m no English major, but do have to do a decent amount of writing for my job. To my understanding, the “we have thought for a while” implies (as you’ve indicated) that you are currently still thinking that they were dating in the past, BUT in no way implies that they are necessarily still dating. They might be, or they might not, but your sentence as described does not tell us for sure one way or another.

@Yeahright I hadn’t read your answer when I wrote mine, or I might not have bothered. You hit the nail on the head!

The sentence is correctly written.

It means 1), or as @Lonelyheart807 describes 2 answers above.

To mean 2), one might remove the word “have”:

We thought for a while that they were dating.

- That means that for some time in the past they thought that. Without the “have”, it doesn’t say anything about what they think now.

@Yeahright I am a bit puzzled by your statement “I do not know how you can assume that they are not currently dating without a time marker or any other context clue that indicates closure.”
How can an action expressed by “the past continuous” be taking place in the present?

We were dating.
We were dating for 2 years.

Neither of the two means that they are dating now. Right? Even if there is no time marker, the past continuous doesn’t convey an action in the present. Unless, we are dealing with deviation or anomaly.

We have been dating (for 2 years). – YEs, still dating.

For me, to get the idea that they are still dating valid, there must be this:

We have thought for a while that they have been dating.

Where did I go off the right track?

What if you replace “for a while” with “for the past year”? We have thought for the past year that they were dating. Doesn’t that imply that the dating is current?

@Ticce
Let me try again, as for some reason, I missed addressing the sentence at the top of the OP:

We have thought for a while that they were dating.

The way it is written implies you don’t think they were dating at all because for a while applies to We have thought [for a while]... (Clause 1.) not to …they were dating.(Clause 2.) which is ambiguous and vague for lack of detail and context.

Do you find the sentence to be correct?

No, it is not because have thought implies that the action of thinking is or could still be going on, but it should not have for a while as a time marker because it is contradictory. …for a while has a beginning and end.

I think it should be We thought for a while that they were dating because We thought… is a past action that started and ended, and for a while meaning a specific period of time that had a start and an end, from that point of view, they match better. But again, clause 2 is very vague it needs more detail to actually be clear and make sense.

Do you agree with my interpretation?

I do not agree with #1 for the reasons stated above.
I do not agree with #2 either because were dating is vague and it lacks a time marker or any other clue to know if they were dating at all let alone for how long.