Not with your setup as it stands. SBS is designed specifcally to be a 'single box solution' for small business environments (hence the name), giving you everything you might need all running on a single server. So it's not unreasonable to lose everything if you take the server down. You could achieve what you are asking if you redesigned the network but then you'd lose some of the advantages that your business installed SBS for in the first place such as ease of administration, low hardware and software costs cost, simplicity of setup etc. Depends how important your request is compared to everything else
There is no restriction on having additional DCs in an SBS network. The only requirement is that the SBS 2003 server must be the DC at the root of the domain (i.e. the main man). You must also bear in mind that the other DCs will only be able to handle roles such as accepting logins. You will not have access to any of the server applications such as Exchange, ISA, SQL which may be installed on the SBS 2003 server.
I personally use RAID 1 (mirroring) in conjunction with other backup strategies.
Yes and if you've got hardware mirroring (using a RAID controller), even better. But I have used software mirroring in the past and it worked OK. You will need to convert your disks to dynamic disks first if you wish to use this option.