I think a user friendly wallet is very important. Currently I have to check, double check and triple check if I want to do a transfer - especially to the exchanges with payment ID. A graphical wallet can reduce errors substantially, where you can assign a new recipient address as well as payment IDs a name and then you can just select the names to do a payment. eg. in the amount block type the amount of coins and in the recipient address you select the name eg Bittrex and then you select the name Bittrex payment ID.
Proving a payment or identity with CN coins sort of against the principles of anonymous coins. If it is possible, the recipient could have a "confirm" option which sends the originator some kind of proof that he got the money, but I don't think it is possible.
I actually think that's GUI wallet/miner are a little overrated. Yes it will bring more people to the coin but there is a chance (and not a small one) that all this people won't be as qualified as one would like. After all, community is always build around 3-4 leaders with a strong tech background and often forum online. I think we should focus on finding those - or becoming those.
Regarding the payment proving - I agree with you, I highly doubt that such functions are possible and if they were possible how would it effect anonymity.
EDIT: Forgot to mention Spekulatius version of slogan - I actually really like it. I was thinking that current version is too long and this one is just perfect. "Merge the future, mine FCN."
Thank you! Going to talk to amphibian about implementing it on website. Maybe instead of "1st merged mining cryptonote coin".