The "cyber-space" is actually part of Ramona's ability. She can get to any place in the world by travelling through subspace. It's why she's the best delivery person for Amazon.ca. It's been in the series since book 1 but rarely used, it reappeared more recently, think it's explained during the fight with Blonde Ninja Girl Roxie Richter and with Knives Chau's dad going all zealous-samurai-dad and chasing Scott around town. XD
As for Ramona dissapearing at the end of book 5, Top Tier has it right... she leaves their place when Scott goes to save Kim. During the fight, Scott was losing steam and was quite demoralized by what Ken and Kyle Katayanagi were saying about Ramona and their relationships with her. So, Kim lied about getting a text from Ramona saying she'll be waiting for him (Scott) and not lose faith in Ramona, that way Scott got a second wind and defeated the Katayanagi twins.
She's not an alien... she's just as bad at staying in a stable relationship as Scott. She bounced from one guy to another, because "the next one seemed better". Up until Scott... although there is still something to be said about #7 (Gideon), as she never really broke up with him properly; I believe she wrote a Dear John letter for him, but it was never delivered.