1. BI is basically SAP Netweaver 7.3 (EHP1) + the Bi content and Bi Content extention add-on on it. You can download them from SAP Marketplace.
I would like to install a dual stack of NetWeaver (Both APAP and Java)
This is not recommended and its not available anymore, you'll need to install ABAP and Java separately.
2. As I mentioned before you can use the Installation guide for SAP Netweaver 7.3 as BI is an add-on.
3. You can install it on both. I'll go for Linux, but its your choice really.
Regards, Juan