i was also blocked when i try to install win7 on my new air,and i thought it was a usb3 problem,same to u.
follow ur thread,i add usb3 drivers to the boot.wim.then a thought came to me,i decide to try add driver to install.wim and see what happen.the result is that what u want,a full solution.
add drivers to boot.wim works for the win7 setup program first running,and after the automatic reboot,i think the stuff in install.wim was incharge.so i tried,and it works.with these wim files replaced,the intall usb stick is albe to intall win7 freely,if you want an shared data partition,u can divide u ssd harddrive under osx with disk tools then bootup manualy with this usb stick and finish installation without keyboard and mouse block issue.
thanks for ur thread and hope u see this.hope it helps.
im from china,here is the link of my solution in chinese:
http://bbs.weiphone.com/read.php?tid=6613405&page=1&toread=1#tpc
too long to translate😝