1. Why the f# didn't they add the option to use also bitcoin addresses (instead of only public keys) to create multisig addresses? I am sure there must be a strong reason but I don't see it at the moment. Many users don't know their public keys immediately, and some (e.g. Circle etc.) don't know them at all.
2. When a multisig transactions has been partly signed, how can the other allowed signatories be notified? (besides emailing them).
If someone can't give you their pubkey then they almost certainly cannot sign for a multisig. You cannot just go and grab random keys from people and expect them to be able to use them in random ways! Thats handing someone a safe and saying "what do you mean you can't open it? it's keyed with a fragment from your DNA; you've got like a trillion copies of that inside you!"; beyond the basic bit of protection from doing something boneheaded there, using hash160s there would basically double the amount of data needed in the blockchain for a multisig transaction; and would much more rapidly run into the redeemscript size limits.
For (2) thats up to the software you're using.
