Well, after a normal install, the iMac would boot to a GRUB bash prompt. rEFIND did not work -I tried that next (installing it from a LIVE LXLE session). Grub boot-repair did not work. I probably should have saved those errors/logs.
I got an error. It turns out, I guess, that it was looking for the x86_64-efi goodness in the (I don't remember exactly where it was ...uh usr/lib/grub I think) in the LXLE live session folders. So, I opened the mounted sda1 (root) directory & copied the x86_64-efi folder. I then opened the LXLE live session usr/lib/grub folder. Open it as root. Copy x86_64-efi into there. The above command then worked.
It worked overall. I Forrest Gumped my way into glory.
Surprisingly, it's not only iMac's problem. It also occures on my HP pc and I'm a little bit confused because after following your solution terminal returned information about missing kernel.img file.