Bought a ASUS ZenBook Pro UX501VW and shrank the Win10 partition via Win10 itself. Made sure there was only three primary partitions and added the fourth for Ubuntu.
Almost every computer that ships with Windows 8 or later ships configured to boot in EFI mode from a GPT disk, and some of what you write later provides further evidence that your computer is so configured. GPT does not have the four-primary-partition limit of the older MBR partitioning system. Thus, you don't need to worry about that limit; and in fact, if you deleted partitions to get under four, you might have deleted something you'll later regret. It's too late to do anything about this now, but I wanted to provide the information about GPT's improvements for your future use and for the benefit of others who might read this.
I had trouble getting the Ubuntu 16.04 Live USB to boot, but doing the "Shift + restart click" made it happen a few times.
It's hard to say what's caused your sporadic problems getting the installer to start, but as a general rule, when dealing with new hardware, I recommend using the newest version of Ubuntu you can; newer versions include newer drivers, which may be necessary to make recent computers work properly. Thus, using 17.04, or at least 16.04.2, is advisable for you.
Marked install third-party, as well as the disable secure boot option, which I assume means Ubuntu should handle UEFI/secure boot compatibility automatically for me.
Personally, I try to avoid using third-party drivers. In my experience, they create more problems than they solve. Of course, you might have a different experience or different needs, particularly if you play games that work better with third-party video drivers.
Disabling Secure Boot makes it easier to load third-party drivers, but this also means that (hypothetical) malware might find it easier to infect your system.
When I make it to the reboot, the MOK Manager pops up. But I have no idea how to make it succesfully make it work, so that I can boot Ubuntu.
This should not have happened. MokManager should appear only if the
grubx64.efi file is not properly signed. Thus, chances are you've run into a bug, but it's hard to say where this bug exists. Broadly speaking, though, there are two possibilities:
- Your firmware -- Your firmware may be buggy in a way that's causing it to work poorly with Ubuntu's Secure Boot features. You might therefore check with ASUS to see if a firmware update is available for your computer.
- Ubuntu components -- An Ubuntu component, like Shim or GRUB, might be buggy. (I consider an improper signature to be a bug, in this context.) This is actually a very broad category, with many possible solutions. I'd start by re-installing without selecting the option to disable Secure Boot. (I'm not positive, but I think that option doesn't actually disable Secure Boot in the firmware; it just tells the kernel to not call back to Secure Boot to validate kernel modules.) I'd also install without the third-party drivers. Even if you know you need those drivers, you can install them later.
If you can't get around this problem by updating your firmware or re-installing with different options, you might try disabling Secure Boot in the firmware. Details of how to do this vary from one computer to another, so a concise description of how to do this is impossible. I do show several examples on this page of mine, so you may want to read it.
I have tried disabling fast boot, secure boot and enabling CSM.
Double-check that you've disabled Secure Boot in the firmware (not just by selecting the option the Ubuntu installer presents). If you have and you're still seeing the MokManager show up, then that implies that something is causing GRUB to fail to launch. You might try my rEFInd boot manager instead. Download the USB flash drive or CD-R version and try booting from it; you should be able to boot Ubuntu and then install the PPA or Debian package to your hard disk. Given the nature of your problems, I can't guarantee that this will work, but it might; and even if it doesn't, details of how it fails may be instructive.
If you want to dual-boot with Windows, do not enable the CSM! At best, this will have no effect. At worst, you'll end up creating a mixed-mode (BIOS/EFI) installation, which will be harder to manage than an all-EFI installation. See my page on the CSM for details.
Only other thing I can think of is that the fourth partition is split in two during the Ubuntu install - OS partition + swap partion. Which could mean that I create a fifth partition and thereby create trouble
No, that's not the issue; as I said, GPT does not suffer from MBR's four-primary-partition limit. Under GPT, there's no distinction between primary, extended, and logical partitions, and a default partition table supports up to 128 partitions. I've got a couple of computers with a dozen or so GPT partitions each and they work fine.