Elderberries seem so eager to root, they may be fine.
Like you mentioned, the conditions seem to favor Fall planting with the rainfall factor. The elderberries here like to have good access to water, and I'm guessing it may be similar with your variety also. I've also had pretty much 100% rooting success with dormant hardwood cuttings, but I haven't tried any other ways yet.
From what I've observed, plants seem to put their resources into the most favorable areas, so I would guess that the growth will naturally slow in the section under the deck and increase in the branches in more sunlight, so I don't think it would be an issue.
Good luck! Interested to hear how they turn out for you!
