Does it really make sense to speak of tense with a modal?  Aren't
most modals invariant?  Doesn't tense require a change of form?
     We must have been there is simply modal perfect.  If must is past
tense, then what do we do with must be there?  The difference between
them is not tense, but aspect. Since perfect aspect gives a sense of
completion, it will seem like past tense, but that is true in all
present perfect verb phrases (like She has eaten.)
     Perhaps historically would was past tense of will, but they seem to
me very independent at this juncture in time.  I would if I could but I
can't so I won't.  Surely all of these have reference to present or
future time.  The difference between I would go and I will go is
attitude about the conditions under which it will happen as opposed to
the certainty of it happening.  To think of one as the past tense
version of the other would be terribly misleading.
     Can we think of used to as past tense if there is no use to?
     The auxiliaries in question are finite auxiliaries, the auxiliaries
which make the verb phrase a finite verb phrase (and an arguable
proposition) by grounding the statement in modality and/or tense.  By
modality, I think we should understand the attitude of the speaker as
being expressed.  It is generally a present tense attitude even if
toward past or future time.
     She has been equals present perfect.
     She might have been adds the notion of uncertainty. It is a
statement about the possibility of it having happened.  The idea would
be that in the present moment of my speaking, I believe it is possible
that she has been.  She should have been adds attitudes about
desirability or obligation.
     We can say she had been, (past plus perfect),  and might have been
(modal perfect) but there is no equivalent for She might had been (modal
plus past plus perfect) or even might has been (modal plus present plus
perfect.)
     We do have periphrastic forms that allow us to combine modality
with tense  and also allow us to compound modality, but generally
speaking the finite role happens only once in a verb phrase.  The finite
auxiliary is also the auxiliary that moves to the front of the clause
when we turn it from statement into question.  She will go equals
statement.  Will she go equals question.  The mood element (grammatical
subject plus finite verb) is at the heart of what makes a sentence a
sentence in traditional grammar, a much better way of talking about it
than the grossly misleading complete thought.
     There may be competing ways to analyze this, but this is the one
that makes the most sense to me.