Yeah, that's basically right. I believe flashprog is also included in libreboot, have a look in the elf/ directory. After that, it should work as expected
Oh you can still do that. The path changed to elf/cbfstool/[TREE]/cbfstool (replace [TREE] with the tree concerened). Note that since the switch to the libre RAM init code, mrc.bin is not included anymore
Their firmware isn't opensource. At least not fully. Only the EC firmware has been opensourced, that is the firmware for the micro-controller responsible for the battery, keyboard, buttons, LEDs, sensors, and so on...
The boot firmware, responsible for getting the CPU and hardware in a bootable state, is absolutely proprietary. There is initial support for one specific framework motherboard in coreboot, a partially libre boot firmware, but it's very new and I don't know exactly what is and isn't supported...
They've been pretty tame lately, but there have been issues historically that made a lot of people (rightfully) mad. You ca read on them here: https://manjarno.pages.dev/
And enslaving a significant chunk of their own folks too.