As for replacing your ubuntu machine, the default distro is 12.04, but I'm sure you can install the latest.
I don't know about the amount of shell tabs you can have (as I use tmux), but my short experiment seems to hold up well.
The only problems I think you'll face is the scary 'OS Verification' at startup and limited memory (but can be solved by using a SD card). croutons are cheap to install so install, remove, install until you find your niche.
Then I just startup a ssh server there. And head back to ChromeOS and SSH to local host (the secure shell plugin is good for this). This means my chroot is totally independent of any chromeos windows or crosh shells.
What software? "The browser"? And do I misread the crouton instructions or is that actually still running the very same thing, same software, in developer mode with chroots for Other Things™?