In complex sentences, whether to position pronouns in the subordinate clause or in the independent clause is largely a stylistic decision. That decision will be based on these considerations: (1) the type of composition, whether narrative, expository, argumentative, persuasive, etc.; (2) the position of the sentence in the paragraph, whether lead sentence or part of the development of that lead sentence; and (3) the function of the sentence, whether a topic sentence, an explication sentence, or a transition sentence.
As a general rule, when a composition is started by a complex sentence with a front-end subordinate clause, that subordinate clause should carry the nouns to clearly identify them to the reader before anything else, after which the independent (main) clause can use the pronouns for those nouns.
Applying this general rule to your first example, the composition should start with the complex sentence in this form:
“
After Juan and Maria finished studying, they went to the movies. Juan was so tired that he went to sleep as soon as they were seated, but Maria stayed awake until the end of the film…”
Starting with a complex sentence using the pronouns in the front-end subordinate clause undesirably postpones the identity of the subjects—a state of affairs that could confuse some readers as to the identity of the subjects (in particular, it could conceivably give rise to the question of whether the pronoun “they” refers to persons distinct from “Juan and Maria”) :
“
After they finished studying, Juan and Maria went to the movies. Juan was so tired that he went to sleep as soon as they were seated, but Maria stayed awake until the end of the film…”
On the other hand, when a complex sentence serves as a transition device in a composition, using pronouns in the front-end subordinate clause becomes not only desirable but functional. See how this applies to your second example:
“John was so keen on getting the gold medal in the swimming Olympics. He practiced long and hard for it, doing several laps in the pool even past midnight in the run-up to the event.
Much to his disappointment, however, John did not win the competition...”
In the last sentence of the passage above, putting the proper noun “John” in the front-end subordinate clause doesn’t work and sound as well in the context of the total narrative:
“John was so keen on getting the gold medal in the swimming Olympics. He practiced long and hard for it, doing several laps in the pool even past midnight in the run-up to the event.
Much to John’s disappointment, however, he did not win the competition...”
For a stand-alone sentence, of course, it’s much more preferable to put the pronoun in the front-end subordinate clause:
“
Though the chicken was spicy, it was delicious.”
In contrast, putting the pronoun in the main clause can make some readers momentarily—and unnecessarily—wonder what the sentence is talking about:
“
Though it was spicy, the chicken was delicious.”
Although it’s ultimately the writer’s stylistic decision whether to put the pronouns in the front-end subordinate clause or in the main clause of complex sentences, I must say that the discerning reader is the best judge of the wisdom of that decision.
