Business casual is fine. It's relaxed, in a semi-outdoor setting. If you're wearing full, formal attire, you will likely be overdressed.
With the exception of Maggie's nieces and nephew, this will be an adults-only celebration.
No.
No. The terrain will be grass/gravel/concrete.
Yes. The venue can accommodate inclement weather.
October is a wild-card month. It could be in the low 50's, or it could be in the high 70's. Pack a light coat/sweater just in case.