Yes it does make it easier, I hope.

Mac must have an interesting boot loader.

If Ubuntu is now on sda, I would try running;
sudo grub-install /dev/sda
and see what happens.

If it "takes" and will boot Mac, the default will be Ubuntu as that is the default setting.

Those settings are in /etc/default/grub. This is where your default OS is set and your time out on the screen menu and the hidden menu option which should be commented out because Ubuntu did detect a second OS (hides the bugger if only Ubuntu on the box).

For some reason the commands seem to have better effect when done from inside the OS.

The only thing I can see that could bugger this is the GPT business at the start of your drive. The only way I know to find out is to try it and see.