Typing the password an average of 3 times doesn't seem like an excessive burden in the context of what you're trying to build.

But that is exposing the passphrase to keyloggers we said.

Wrong: Keyfiles are an alternative to passphrases. Although they are named "key"files, they have nothing to do with the actual "key" but function exactly like passphrases. They are not in RAM the whole time, but are accessible through the local filesystem. If this is better or worse, your decision. If you run your browser with a different user account using

you could protect the keyfiles from being read by the browser.

Both cryptsetup-luks and truecrypt can use keyfiles instead of passphrases or in addition to passphrases.

Good idea, running the browser with another user account so as to hide the keyfile from malware coming from the browser.

We can do even better than that it seems with SELinux, otherwise it would not have been invented.

Yes, SELINUX is probably even better if configured right. Though it needs more research to configure it right.