My question relates to hypothetical situations in the past; however, I am looking at it from a narrative perspective, where the outcomes of past events are presented as uncertain.
Traditionally, to hypothesise about an event that didn't occur in the past, we use a combination of the past-tense auxiliary 'had' followed by the modal auxiliary 'would' and the bare-infinitive auxiliary 'have':
 If you had studied, you would have passed the exam.
This is often called the 'third conditional,' which is discussed in the following article: https://www.grammar-monster.com/glossary/conditional_sentences.htm.
Conversely, we use the so-called 'second conditional' to talk about an unlikely event that pertains to the current moment, not a time frame that has passed. This construction uses the past-tense form of a lexical verb and the modal 'would' without an auxiliary as complement:
 If you studied, you would pass the exam.
These are quite simple concepts, but something that is less clear is which construction to choose when writing a past-tense narrative. Though these stories are presented in the context of the past, the events are effectively transpiring as they are read.
See these next two examples, and consider what sounds more logical:
 He thought about carrying the box down the stairs, and if it were lighter, he would. Alas, it was not, so he left it behind.
 He thought about carrying the box down the stairs, and if it had been lighter, he would have. Alas, it was not, so he left it behind.
In Example , I use the 'second conditional.' The intention is to show that the opportunity for him to carry the box is still on the table, but the likelihood of him doing so is low, as it is too heavy.
In Example , I use the 'third conditional.' This implies that the situation has passed, and the opportunity to carry the box is no longer available.
The problem with Example , as you might be able to guess, is that it implies that the situation involving the box has ended, but we can clearly see that it hasn't. The sentence following the 'third conditional' explicitly references the box in the past tense, not the 'past perfect.'
Because of this, I would gravitate toward using the construction in Example , but I have reservations. The 'second conditional' is often used in the context of the present to hypothesise, so it almost sounds as though we have changed tense or are speaking about a future event in the past (the latter of which would be the case if we changed 'is' to 'were' and 'will' to 'would').
Grammatically, I know that 'would' is the past-tense form of 'will,' so this shouldn't be an issue. However, I would prefer to have a second opinion.