a) Human beings are not good at picking random phrases so don't create your own brain wallet passphrase.
This is a valid point. While I do know, that human beings are bad at creating randomness, we tolerate this weakness for practical purposes: Or don't you have passwords you chose by yourself? But the longer your self-chosen password / passphrase gets, the more it compensates the weaknesses of its non-randomness. Thats why I came up with the last question in my OP: "How many words do you think you have to collect with this method to meet standard-brain-wallet-security?"
b) Don't reinvent the wheel - use electrum
Electrum generates a random seed that is basically 12 words. You can memorize those or write them down. They are all you need to restore your wallet with multiple addresses. Electrum also supports watch only wallets and offline wallets. So if you want to create a brain wallet use electrum. It is all you'll ever need.
I will have a closer look at alternative clients and they will have a place in my bitcoin-environment. But software-solutions don't qualify for my "total-foolproofness"-standards.
As a second level of security, write this passphrase down and then split it with shamir's sharing secret into 5 pieces with a quorom of 3. Hide them in different places. If you ever forget your phrase, you only need to retrieve 3 of them to recreate your passphrase and use that to regenerate your private key.
Cool! I never heard of that! This is definitely a good idea if you want to give your bitcoin-credentials to friends.