The statement "Who cares?" is often seen as extremely rude and even arrogant. Let's look at a more specific case: consider George W. Bush's response to journalist Bill Hangley at a press event in 2001:
Because I had serious misgivings about the president’s performance to that point, my own involvement in the whole operation had left me feeling a bit like a pseudo person, so when I had the chance to shake Bush’s hand, I said, “Mr. President, I hope you only serve one term. I’m very disappointed in your work so far.”
His smiling response was swift: “Who cares what you think?”
This is a very small thing for a man in a position of importance to say. Although Hangley had criticized the president to his face, a man who was more sure of himself might have replied generously, even magnanimously: "I'm sorry you feel that way," perhaps, or "I am doing my best to change your mind." That sort of thing. Instead, what came out was rude to the point of arrogance. It is the statement of someone in power to someone who is powerless.
Granted, "Who cares what you think?" is more specific and pointed, but the general "Who cares?" amounts to the same thing.
But the statement is not always rude. It may also be a weak response to an accusation:
A: You know smoking will kill you, right? Yet you do it anyway.
B: Who cares? We all have to die of something.
That usage suggests a forced insouciance, a resignation in the face of the perceived powerlessness of the person to quit smoking.
It can also be triumphant and expansive, even exultant in a way that is not at all rude:
A: The weather report is saying heavy snow is expected next week.
B: Who cares? You and I will be at the conference in Hawaii that week!
3) will for present habits:
There is no question that the verb (or verbs; there may be several) will is one of the very trickiest ones in the English language for foreigners ever to master. The deontic senses are seldom intuitive to a non-native speaker. I strongly advise you to carefully study the OED’s entry for this word’s incredibly many subtle uses.
In this case, your two examples are not of the same thing at all, and you have mischaracterized them. The first uses will to express habitual action; it does not indicate a simple future situation. This is the OED’s sense 8 for this verb:
8. Expressing natural disposition to do something, and hence habitual action: Has the habit, or ‘a way’, of ––ing; is addicted or accustomed to ––ing; habitually does; sometimes connoting ‘may be expected to’
This is related to sense 15, which is still not a simple future, albeit perhaps closer to that:
15. As auxiliary of future expressing a contingent event, or a result to be expected, in a supposed case or under particular conditions (with the condition expressed by a conditional, temporal, or imper. clause, or otherwise implied).
Your second example, the one about the car, is completely different. This corresponds to OED sense 12:
12. With negative, expressing the contrary of senses (def#6), (def#7), (def#10), (def#11): thus commonly = refuse or decline to; emph. insist on or persist in not --ing. Also fig. of a thing. (See also (def#9), (def#13).)
Here, your car is persisting in not starting. It is the figurative sense at the end extending to things, as though they had the will to refuse. The referenced senses 9 and 13 are respectively:
9. Expressing potentiality, capacity, or sufficiency: Can, may, is able to, is capable of --ing; is (large) enough or sufficient to.
15. As auxiliary of future expressing a contingent event, or a result to be expected, in a supposed case or under particular conditions (with the condition expressed by a conditional, temporal, or imper. clause, or otherwise implied).
As I said, will is quite complex. Please study standard reference works regarding its use.
Best Answer
See this definition from Webster.com
I think the answer you quoted in the other question is not really correct. "Excuse me" is not generally used to ask for permission. The example you give is of one of the following definitions:
They're basically introducing the request by apologizing for the interruption.