Hi spin! A few comments:
1.
A: 1-2 hold firmly to the word
This doesn't give any credit to the first verse:
Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand
Skipping right to verse 12 seems to pass over the purpose of verse 1, which seems to be that Paul is going to summarize what he had first told the Corinthians with regard to his gospel of the resurrection of Jesus. So, it seems to me there should be SOMETHING expected after verse 2 and before verse 12 along those lines. Ben's Marcionite reconstruction makes more sense to me than nothing at all.
2. Verse 20's pronouncement that 'In fact Christ has been raised from the dead' falls flat for me without 3-11 because it is missing a reason for such conclusion. How can he say that in FACT Christ has been raised without saying anything about WHY they believed that? (side-note: it appears to me that the Corinthians were not questioning whether Christ was raised - they believed it for SOME reason that Paul had preached, but that some questioned whether 'normal' humans could be raised and/or how).
3. Nice observation on the chiasm. However, I don't see one as strong or complete as you do because I see it as a natural way of argumentation and of just expressing things, and in this case verses 20-24 are more of a recitation of the expected chronology of events than a purposeful chiasm:
Christ was the first human raised
In the future Christ will come and others will be raised
Then He will judge others - and death itself will then end.
After that Christ will submit to God having completed his mission
Should we conclude that Paul intentionally created a chiasm within a chronological rendering of expected events?
Without 3-11 it looks more like this to me:
A. Intro/promise of a coming reminder of what was taught
B. Encouragement to keep the faith
C. Implications of no resurrection
D. Christ was resurrected, and the chronology of future events including resurrection of believers
C. Implications of no resurrection
B. Keep the faith - stop doubing/sinning
E. What is the resurrected body going to be like
With 3-11 or some other kind of explanation then a second 'A would be inserted between B and C.