None of both users worked, duh -_-
My iMac supports USB booting (only UEFI on Apple's EFI, also BIOS with rEFInd), so in theory, it should just work (as Trisquel 7 also has an UEFI boot manager). However, I couldn't start from my USB, after burning it using dd, as said earlier. So yeah, maybe debootstrap. I have little experience with the Debian installer disk tools, so if that could work... :P
