I think the Luke/Leia thing very obviously was foreshadowing, or at least a hint. Maybe they hadn’t decided definitively that was the road they were going down yet, but this more than conveniently left a strong opening for it. No one will convince me it wasn’t at least thought of at the time. The fact that she alone senses Luke in a very similar way to Vader in the same scene is too much to be a coincidence.

As for the Lando and Han line, pretty sure it’s only to emphasize how similar the characters are in terms of personality and background.