There isn't really any vagueness about when to use must and when to use should.
Must always implies absolute obligation or certainty.
Should always implies a request, suggestion, or expectation although in some contexts a request may be so strong that it could be seen as an obligation.
See @Cerberus's excellent answer here exploring the "built-in slipperiness" of English words commonly used in the general area of volition/expectation - where must applies to both at the extremes of obligation/certainty. Things only really get murky at the lesser levels.
But OP's example 2 is structurally ambiguous - You must cross the street to get to that store could mean any of...
You are obliged/I order you to cross the street [and thereby to get to that store]
You would/will have to cross the street if you wanted/need to get to that store
More naturally we interpret the whole sentence as informational, with an implied if you want to get to the other side. The obligation implied by must doesn't come from the speaker - it comes from the laws of physics which say the only way to be on the other side of the street is to cross it.
Note that in practice, people often deliberately or unwittingly flout these distinctions. I'm sure the vast majority of people who have ever said, for example, "I must be mad!", or "We must have dinner together soon" didn't really mean they were absolutely sure, or imposing an absolute order.
Best Answer
"Figuratively" should never be used to denote strict meaning. "I figuratively killed that guy" means you didn't actually kill that guy, but rather killed his character in a video game or beat him handily at some activity or something similar. You're indicating that you're using a figure of speech, and are not using the literal meaning of the words. Most of the time, "figuratively" can be left out; whether or not you did literal killing should be clear from the context.
Likewise, "literally" should never be used to denote anything other than strict meaning. "I literally killed that guy" means you committed murder or manslaughter. It's often misused to add emphasis; for example, comparing the figurative killing to literal killing. Such a comparison doesn't make sense, since doing a super combo on a game character is nothing like murdering your friend. This is apart from scenarios where it would be entirely inappropriate to draw the comparison. (If you beat someone at a video game and for whatever reason they become distraught and take their own life, you probably want to avoid saying "I literally killed that guy.") Thus it's better to leave "literally" for its intended purpose.