No, you can't boot into ARM code without it. ARM part is disabled power-on, VPU is a general purpose processor really. Basically, on start-up it loads its start.elf (i.e. boot application compiled for its instruction set) from boot-media, initializes some hardware, then loads ARM boot image into common memory, then starts ARM. It also exposes some low-level hardware interface via syscall-type "mailbox" interface.

It looks like, per https://github.com/librerpi/lk-overlay#what-features-work , yes you can boot to Linux on a Pi 2 with composite video and ... it doesn't use the word headless anywhere, but I'd be very surprised if you can't just omit video outputs completely.

EDIT: Actually, reading more carefully it looks like there might be more than one blob and it's not 100% clear to me which this replaces, so now I'm less sure that you can boot without any proprietary blobs. I'm not sure that you can't, but I can't tell.

There’s also the serial console one could use