True. And, as far, as I understand, it will never be. Although, depends, what you calling RTOS in this case: “firmware” (bootloader), which loads your sketches, or default “sketch” with basic phone interface. I just talked about first one. THe second one, is kinda open source an lays on the github.
And I very very like to see (and patch) the sources of first one, and add some missed features there.
Unfortunatelly, true. At least, documentation. While there is ports of Arduino IDE under GNU/Linux somewhere in neighbour thread. There is native Eclipse IDE on GNU, but I doubt about plugins.
And, unfortunately, there FirmwareUpdater tool, which is only available for Windows.
Depends what do you want about it. Not sure about “byte-to-byte” (patch-to-patch) one, but you can either use Official ARM Toolchain, which is shipped with both binary and sources form (although, there is broken libc.a in latest release), or, if you’re using gentoo, use “crossdev” tool to prepare arm-none-eabi toolchain (will require patched newlib ebuild).