1. There is no default user/password per se. By default, BUILTIN\Administrators is granted sysadmin role, so any local administrator will be able to log in.
2. Make sure that SQL Server is configured to listen on TCP/IP for a start (use SQL Server Configuration Manager)
3. You should be able to, other companies had. I haven't done it myself, so don't know the specifics, but presumably you would call a silent installation of SQL Express from your install program. After it is installed, run whatever CREATE scripts are required.
MARCUS. Why dost thou laugh? It fits not with this hour.
TITUS. Why, I have not another tear to shed;
--Titus Andronicus, William Shakespeare