Yikes. With study buddies like that, who needs study enemies?
You are completely right in your justification for eliminating (D). Try to remind your friend that the standard of right/wrong on this type of question is just, "Did it happen? Did the author do this? Can I match this answer choice wording to the wording of the argument?"
So, as you said, there is no way to match up "someone's actions
resulted in harm". The author's conclusion is in future tense. Again, LSAT is testing us on what was said, not what can be inferred about what might ultimately be the case.
And, perhaps most crucially, the author never uses any value-laden term we could match up with "irresponsible".
(D) basically describes the argument as this:
Premise -- Damon's actions resulted in harm to others
Conc -- Damon's actions were irresponsible
Neither one of those is an accurate match for what was said.
(E) describes the argument as this:
Conc -- Two situations can't both occur (Damon will be forced to choose between professional and family responsibilities)
Premise -- Something necessary for one situation is incompatible with something necessary for the other situation (fulfilling professional responsibilities requires working on the paper all evening without interruption, but that's incompatible with making time for family responsibilities, seeing his daughter's recital and taking her out for ice cream)
=== other answers ===
(A) The author doesn't single out one of Damon's options as more likely and conclude that the other one cannot occur. Rather, the author presents the two options as mutually exclusive and concludes that Damon will have to choose one at the expense of the other.
(B) This choice describes an argument by analogy. But there is no analogy here, just a description of two choices that are mutually exclusive.
(C) The author never invokes sympathy or excuses Damon. The author is merely describing in neutral language the way that Damon has got himself into a jam.
Hope this helps.