What a cute pattern! I think either of your choices will work well for this pattern. The alpaca will make Imagination much drapier than Stroll would be, alpaca does that. Either would block out well, but mainly Imagination would make it have more drape and Stroll would have a bit more spring and bounce to it. Also, Imagination will probably pill more than stroll since the alpaca fiber is fuzzier than Stroll which is much smoother. A general rule of thumb, the softer something is, chances are the more it will pill. I think it is a bit of a give and take when comparing these two yarns - Imagnation will have more drape but the Stroll will pill less. Depends on which quality is more important to you.
Hope this helps a bit, be sure to show pics when you are done, it is super cute :)
Are you wondering how much yardage you'll need when you make it with laceweight yarn instead of DK? There is no real way to know exactly. You could order the amount (or a little more) of yardage that a comparable sized shawl would take (similar laciness and needle size). The problem I can foresee for you is that the shawl will turn out much smaller made of laceweight than DK. Is it a pattern that has an easily customizable size? What I mean is- can you add repeats to make it larger? Just some thoughts.
(Love your id, by the way. :D)