Learn English – When should you use “then” and when “than”

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.

Breaking this down:

• and/or is as official as English gets in the sense that you can use it in extremely formal contexts. There is typically a better way to say whatever is being said but it does convey a specific meaning.

• You should use and/or when both options are applicable in its place. "I would like cake and/or pie" means "I would like one or both of the following: cake; pie."

• The main reason for using and/or is to remove the ambiguity of whether and means "only both" and whether or means "only one." And/or explicitly means "it could be one of these or both of these."

• The confusion is drastically exacerbated by mathematicians, logicians and/or computer scientists who are very familiar with the differences between the logical operators AND, OR, and XOR. Namely, or in English can be either OR or XOR; and/or can only mean OR. As you may have noticed, all of the terms look similar which leads to the confusion in parsing sentences like your title.

EDIT: To strictly answer the question, you should use A and B when you explicitly mean both A and B, and you should use A and/or B when you mean A or B (or both).

In response to a request for pronunciation, I typically treat the / as a hyphen and simply say "and or". This is not always standard for the / symbol, however, and other words or phrases with a / may be different.