FAT32 isn’t used by default in any version of Windows that is currently supported. It’s only default for very low capacity USB flash drives. Windows 10 will default to exFAT for most USB flash drives and NTFS for all SSDs.
The more likely reason for Blizzard stating this is the “required minimum” is the sheer number of complaints they get about slow loading screens and loads in general with regard to CASC enabled games. After so many patch cycles, CASC gets fragmented enough that it really bogs down on platter based drives and the only workaround, and only in the short term, is to uninstall and reinstall or do a double-clone of the drive where you clone to another drive temporarily, format the original drive, and clone back to the original, since this will force files to be written in as contiguous a manner as possible.
On the Mac side, SSDs are essentially required because of the combination of APFS’ enumeration problem combined with CASC’s fragmentation issues. SSDs are literally the only thing propping up APFS right now, and even then players have to reinstall the game from time to time simply because the enumeration + fragmentation issue is bad enough that it can bring even mighty NVMe drives to their knees. Remember, all SSD drives’ Achille’s Heel are the random 4k R/W IOPS, especially at Queue Depth 1. This will be the case until I/O workloads become properly multithreaded, which they haven’t yet for most OSes and apps outside of specific benchmarks.
Yes, you actually can. It’s not easy to do, but it is possible. But the only reason you’d do that is if you were using it as a data drive in a Windows XP machine. Outside of that, there is literally zero benefit to doing so.
At least your desktop meets the requirements as it has a GTX 970 in it. If your laptop supports an eGPU, that could be used, but I don’t think it does given its age (and the fact almost nobody outside Apple used Thunderbolt 3 for the longest time).