1) In mtgo, i don't think there are any options to play vs a computer. you can however, play vs other people on mtgo without spending tickets
2) I'm not the best person to answer this as my actual mtgo tournament experience is close to nil. That said, it's general knowledge that the level of tournament play on MTGO is fairly high.
I can give you some resources/links if you want more info about this. A fair number of JSP'ers are also knowledgeable in this regard, depending on what formats they play.
Ticket costs: check this link i think it's updated.. ($1 = 1 ticket)
https://wizards.custhelp.com/app/answers/detail/a_id/683) Rewards? same link above
4) generally, no

5) yes, with tickets you can do almost anything (they're mtgo currency)
6) yes, you can sell cards back for tickets or even for cash (or fg as the case may be)