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.
Nowhere would be the normal usage. However, it might be appropriate if you had done a search of a house for something or someone, to shake your head in despair, and say "It's not anywhere". The implication is that you have looked everywhere, and it is not in any of the places you have looked.
It is slightly idiomatic, though, so it is not for use in formal writing.
Best Answer
"Staff" and "staffer" are essentially the same, but they are slightly distinct.
"Staff" is defined as:
Therefore, "staff" is a collective noun (such as "family" or "crew").
"Staffer" is defined as:
Therefore, "staffer" refers to an individual member of a staff.
Once you account for the fact that "staff" is collective (as you did in your sentence), the sentences' meanings are identical. Personally, I would use "staff" because it is much more common [Ngram].