This doesn't affect the author's argument either way. The author's argument is about average turnaround time — i.e, the average time it takes to respond to any one top priority emergency — and not about the number of such emergencies.

In other words, to see how this answer choice affects the argument, it doesn't help us to know how many heart attacks and strokes there were — we would need to know something about the turnaround time for these emergencies. A decline in the number of heart attacks and strokes wouldn't bring down the average turnaround time unless the turnaround time for any given heart attack or stroke were also generally lower. Since this answer choice tells us nothing about the average turnaround time, it neither strengthens nor weakens the argument.

This might be trying to "strengthen" the argument by suggesting that the mayor is a fan of redefining things in general. But that's close to an ad hominem argument, and doesn't strengthen the logic of the argument.

Whether the mayor's redefinition of "top priority" is "right" or "wrong" according to experts isn't relevant. What we want to know is if that redefinition — whether or not the experts like it — is the actual reason for the apparent reduction in the average turnaround time for top priority emergencies.

What other cities include as "top priority" isn't relevant. We just want to know whether this particular mayor's redefinition of "top priority" — again, regardless of whether other people define "top priority" the same way — is the real reason for the apparent reduction in turnaround time, as the author claims.

This is correct. Don't get thrown off by the apparent math involved with the "one half" here. Remember that we just need to know whether there were enough gunshot wounds and electrocutions last year that those emergencies brought up the average turnaround time, suggesting that excluding those emergencies would reduce the apparent turnaround time.

In theory, even a few gunshot wounds or electrocutions would be enough to do this, assuming that the makeup of the other "top priority" emergencies doesn't change much from year to year. But knowing that a very significant proportion — half — of last year's emergencies were these time-consuming gunshot wounds and electrocutions very effectively strengthens the author's argument that removing these situations from consideration is responsible for apparently reducing turnaround time.