You can. You might run into some hardware headaches, though, since everyone's running Windows 10 these days and they don't really bother making drivers for the older versions anymore...
Are you trying to install from a USB? If you're using a 3.0 port, it probably won't work because Windows 7 doesn't have native support for 3.0 during the installation process...
If you have an email account set up via IMAP in Microsoft Outlook, those messages should be getting deleted automatically—they’ll head straight to the recycle bin, which Outlook clears out on its own after about 30 days.
I’m honestly not sure which band it uses. Since most phones here run on GSM while CDMA is still pretty common across the US, you really want to double-check before you pull the trigger on a purchase.
Just grab Windows directly from the Microsoft website, wipe your hard drive, and start completely fresh. Honestly, you should ditch that "pre-activated" junk...
No, you’ll just end up paying for more iCloud storage to save your files; there isn't any way to actually increase the physical storage on your iPhone itself.
I haven't heard anyone say exFAT is unreliable. If you aren't planning on splitting things into two partitions, your best bet might just be picking up some software that lets your Mac write to NTFS...