I need help understanding the structure of Spenser's Amoretti Sonnet 75, and how to compare it to the structure of Shakespheare's Sonnet 18.

Spenser's Amoretti Sonnet 75 is
structured with three quatrains (four line stanzas) and a
couplet to comprise the fourteen lines of the sonnet form--as prescribed by the original
sonneteer, Petrarch--in the Spenserian rhyme scheme
ababbcbccdcdee (abab bcbc cdcd ee), with linking of rhyme/thought at concatenated lines
bb cc (4-5, 8-9). Starting in the middle of the sonnet, the third
quatrain introduces the sonnet paradox: "you shall live by fame; / My
verse, [you] ... shall eternize." A paradox is an idea or concept that appears to be
false ("baser things devise / To die ... but you shall live"), but is actually a truth
of some sort: "My verse, [you] ... shall eternize."

The
concluding couplet (two rhyming lines) resolves the
conflict between mortal death and immortal life by explaining "Where whenas" all in the
world shall in due time die, their love (the sonneteer and the lady with him, Elizabeth
Boyle) "shall live" and be read by all through all time in his Sonnets, which will give
renewal to all those who read them.

Quatrains
one and two are a little trickier to analyze for structure.
Spenser innovated a form of sonnet that allows for a
logical progression of one subject without the introduction of a paradox. This is
different from the Petrarchan form because it and the Shakespearean form have three
subjects in all, one in each quatrain. This Petrarchan/Shakespearean form allows
for two points of view and a
paradox.

In contrast, Spenser's
innovation allows for one subject carried through all three quatrains for
a logical progression with one point of view and without paradox.
The analytical question is: Does 75 represent the logical progression of one subject in
the form of a conversation or does it represent two points of view and a paradox before
the resolving couplet?

Having defined the question in this
way, it seems more clear that Sonnet 75 introduces two points of view and a
paradox before the couplet. This means that, structurally, there are two
voltas (turns in the subject), one where the points of view change (line 5) ("Vain man,
said she, that doest in vain assay,") and one where the paradox is introduced (line
9):

Not so,
(quod I) let baser things devise
    To die in dust, but you shall live by
fame:

This analysis makes
this sonnet structure the same as
Shakespeare's Sonnet 18: fourteen lines; three quatrains;
two voltas, lines 5 and 9; one paradox, quatrain 3; and one paradoxical resolving
couplet, except for Shakespeare's unconcatenated (un-linked) rhyme
scheme of ababcdcdefefgg (abab cdcd efef gg):

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So long as men can breathe or eyes can
see,
So long lives this and this gives life to
thee.