Windows – Apple software upgrade crashes (heap corruption in msiexec.exe) on windows 10 bootcamp

I have a mid-2012 15" retina Macbook Pro, and I've been using bootcamp with windows 7/8/8.1 for years without any drama. I've installed/wiped/re-installed bootcamp dozens of times, so I'm quite comfortable with the process.

Recently I made the stupid mistake of upgrading to Windows 10 and everything has gone to hell. Even more stupidly, I upgraded to windows 10 before updating my Apple Drivers. (My OSX is up-to-date yosemite).

My problems:

1. After windows 10 finished installing, hardly anything works in windows. No advanced trackpad facilities etc. and most critically, no wifi to download the required drivers!

2. In windows, I connected to the internet via ethernet (luckily USB ethernet worked) to run Apple Software Update. But to my dismay after downloading the 1.2GB download, it starts to install, but then freezes for ages, and then crashes with a message "This application has caused an error. close or debug". I tried about 5 times, same thing every time.

3. In OSX, I tried to download the Apple Support Software via Bootcamp Assistant onto a USB drive. BUT no matter what USB drive I try (I've tried 1x64GB stick, 1x32GB stick, and 1×32 CF card), I get the error "you need at least 6GB, you have only 4.75GB). And these are freshly FAT formatted, single partition, MBR drives. I can't even download the drivers on OSX!

I know I was supposed to update my apple drivers before upgrading to windows 10. But I forgot, and that was a mistake, but surely there's a solution now which doesn't involve wiping the whole partition and re-installing windows from scratch? (or chucking the laptop in the bin).

UPDATE:
I just tried again and it crashed again. This time I chose to 'debug' in VS2015. I can see the msg "Unhandled exception at 0x00007FF9CA54DFAC (ntdll.dll) in msiexec.exe: 0xC0000374: A heap has been corrupted (parameters: 0x00007FF9CA5A22B0)."