What's my responsibility in a car accident for repairing previously incurred damage?

You are only legally liable for the damage you yourself caused--not for damage done previously by others. As a practical matter, proving he who is responsible for one set of damage vs. another may very difficult. For example, it may be impossible to determine, even with the best of intentions on all parties, which hit or accident damaged her alignment.

If you have liability insurance that would cover this, your probably should report it to your insurer and then it's their job to determine who did what damage and what ought to be paid. If you don't have liability or are determined to not report it, you might want to get a sense for the cost of repairing the other items; if it would add little to the costs you're willing to pay, you may wish to consider picking up some or all of them to avoid potential litigation.

No good deed goes unpunished as they say.  No, you are responsible for the damage YOU did to the car not the prior damage and certainly not the alignment, unless they can pin it on you.  The problem here is going to be that it is going to be her word against your word and proving that you did all the damage may not be as difficult as you think (except the alignment thing).  If the damages was done close in time it may be difficult to differentiate the dents.  In other words, say the car was hit a month before.  Chances are that the damage would have already begun to rust or chip or something to show that it was older than your hit.  But here it may be difficult.  Nonetheless do not lay down and die on this.  Stand your ground.  Good luck.