"If you have" is asking about a concrete situation. "If you have one day to live..." would generally be interpreted as asking about "some day, in the future, when you have only one day left to live, what will you do?"
"If you had" is asking about a hypothetical situation. "If you had one day to live..." would be interpreted as "If, right now, the doctor told you that you had only 24 hours to live, what would you do?"
Your 2b option seems to be confusing the present subjunctive mood with past subjunctive mood (and subjunctive mood is already confusing in English). You need to select whether you're talking about the present, or the past, and ask accordingly.
Present subjunctive:
If you had one day to live, what would you do?
Past subjunctive:
If you had had one day to live, what would you have done?
Your reasoning is on the right track, in that "had been broken" often would be read as implying agency or intent. However, had broken is perfectly correct here. In fact, you do not go far enough: if the article had said "had been broken," most native speakers would assume that some individual had broken it. There are some definite subtleties here, however.
First, let us recall that some verbs can be both transitive and intransitive. Broke is in fact one such verb. Either a subject can break an object, or an object can break, similar to how one can eat dinner or simply eat. However, since the subject is the physical recipient of the action in the first case, it is not so easy to see.
There are two ways that this sentence could be written, with three interpretations.
The coupler had broken.
Really, this the past perfect way of saying it broke. No agency implied.
The coupler had been broken.
In most contexts, this would imply that some human had broken it.
However, in this context, it would not:
The coupler had been broken for a while.
In this case, broken is merely an adjective, not a past participle, and thus refers to a state, not an action. No intent is implied.
That said, this is Indian English, which is dialectically distinct from the version I speak, so certain rules might be a little different.
Best Answer
You have clearly misunderstood the usage of modals and their time reference. Incidentally, I shall explain it to you with some examples before I jot some points down on the paper.
To start with, modality is a branch of linguistics which enables and allows us to express, or emphasize the (in)ability, (in)capability, (im)probability, obligation, (im)possibility, permission, necessity, (un)likelihood, (un)certainty, and so forth of an action or event.
Modal auxiliaries, often referred and known as 'modals' or 'semi-modals', in the English Language are:
e.g.
will - expresses assumption, disapproval, politeness, habits, or even certainty of the present
had better - expresses strong advice or order
need - [usually negative] expresses immediate necessity or, sometimes, things that are not necessarily true
........
A majority of these modals have their perfect variants that link the previous event with this/that point of time. Furthermore, perfect modal auxiliaries are formed by the following method/pattern:
But they don't necessarily refer back to the past; in addition to that, they can describe the completion of a future event, that is expected/considered to be completed before some action, as in:
As I was saying,
To return to the previous point, although most of the modals are not backshifted in reported speech such as : should, would, had better, needn't, and others; they mean the same in the reported sentence as their meaning was the same in the non-backshifted sentence.
CONCLUSION:
To wrap this up, your sentences do differ in terms of their aspectual meaning.
WHEREAS
Quick have-to semi-modal tense reference:
I hope I provided you with elaborate information on this subject.