Q:
Mac beta Characters
I don’t think characters will be reset, since they will be playing with the “regular windows characters”. Also, only the client is mac-based, the account can be used for either windows or mac, so I’m assuming existing accounts that played with windows could simply download the mac beta client with a mac and play their characters there. As for the client itself, I can’t give a definite answer on that, but if it’s the same as the windows client, you’ll just get an update most likely.
If you have any other questions, I would suggest you use the GW2 Support.
http://en.support.guildwars2.com/app/answers/detail/a_id/9231
Your characters created using the Mac beta client certainly will not be deleted – you’re playing the live game along with everyone else, just using a client on your end that’s still being tested. If you were to log in to the game using a Windows client, either on a PC or under BootCamp, you’d see the same characters.
As for you second question, only ArenaNet knows the answer, but the experience in the betas on the PC side was that the same game client was updated; there was no need to download the whole shebang all over again. I would expect the experience to be the same on the Mac side, once that client leaves beta.