The most recent ones (last 25 or so) may still be recorded in your SIM, but there are no SIM tools for the N900. If you can access another/older phone (like a 6230) you may be able to get that phone to read the last couple dozen SMS from the SIM. Your service provider may also be able to provide a listing of them as well (or on their website).

When you restore from a backup, you're telling the system to delete any changes done since the backup was made. That includes SMS. Why is that confusing or unexpected?

I would suggest you use a backup logging tool (like glogger, or OVI Suite) to backup your SMS if they are that important to you.