How do I move the cursor past the end of the line in Insert mode when using vi-style line editing on bash?
It turns out the solution is simple: update bash to 4.3. The default bash on Macs (even Yosemite) is 3.2. (My question above says I did update and it didn't work, but I think it took a while for it to "take.")

Install Homebrew with

ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"


then do

brew install bash


then change your default shell path (in Terminal Preferences) to

/usr/local/bin/bash


and that's it. Check your bash version with

bash --version


and as an added bonus, you can get mode indicators!

bind 'set show-mode-in-prompt on'


and it will show a + for Insert mode and : for Command mode.