A) yes, your information is save on a "server in the sky" (Internet host), you can access your data via a web browser and manipulate it it there; I can only speak for Mobical.net in this regards, I've been editing my data online and on the device(s) both. I use the web interface for more "bulk" edits (importing large calendars, mass changes to people, etc.). I also use the web interface to download backups of my data to store on my laptop just in case (can never be too careful).

B) no, the solutions using SyncML are all over-the-air, no connection to a computer necessary (which is the whole point! ;) ). You will need to have a data plan enabled for your BlackBerry with your carrier in order for it to work; the connection works fast enough over an EDGE connection so it's not that bad. The first time you sync the two ends together takes the longest (for the obvious reasons), but beyond that the normal syncs are fast and painless. With the Nexthaus client you can even have it constantly syncing in the background and never have to 'launch' it if you want. I prefer to launch it by hand, though - I'm not a fan of automatic type things.