Yes this is completely free to play. Every user gets a free region, which you can access by clicking on the 'Home' button. If you require more regions, you will have to purchase land.
Yes, you will need a mouse or touchpad.
Right now you can only access sinepsace with a PC that has Windows/Mac/Linux. We are working on android and IOS clients, you can access them on request (experimental versions). We are planning to do a full release in the future.
To see upcoming changes or news, visit our blog. For talking to our commmunity or to check out our experimental features, visit our forums.