Why don't you ask your Japanese friend to explain you how to register?
What I can see here is that he isn't a true friend!
Just register WITHOUT the mobile service!
You can do that later on after your in ok!
Your registration is getting hard because you want all at the same time and that can't be possible my friend even in real life!
So mixi can be register with only webmail address!!
Btw my friend is a female
Since last month, she is on long journey travel and really hard to get internet access and to find a computer that have Japanese input. So it will take some time to get her reply.
Ulver_684-san, kindly can you invite me to mixi?
I already made a stupid mistake by randomly put a mobile phone address and somehow cannot change it through my friend invitation link