Is it only Aikido if it comes from Ueshiba???
Yes. If not, it's called Aiki-Jujitsu or Aiki-Jutsu, and it's probably extinct over a century ago.
Not necessarily. There are other arts that have used the name "Aikido". Even many Daito-ryu dojo that have used the name. Ueshiba's decendents are the most numerous
users of the name, but not the only ones.
Just go to practice at Aikikai Hombu dojo once and tell me if it was "polishing the mirror".
Depends whose classes you go to
. Off and on I think that I spent 2 or 3 years training at hombu, although I don't get there much these days. There's a fairly wide range of styles and practices.