Maybe, maybe not. But you should try and see if you can write a script using the VirusAction feature (found in version 0.9 and later). It allows you to run an arbitrary program whenever a virus is found.
No. Version 1.5 was capable of this, but it broke on certain operating systems.
ClamSMTP is designed as a proxy, not a mail server. So having it listen on a higher port and directing traffic to it is how it's meant to be installed. That said, if you'd like to work on a fix for this, I'll include it (after proper testing and all that goodness).
This should be fixed in versions later than 0.9. You may also want to try upgrading your version of clamav. If it still occurs then let the list know.
When clamsmtpd runs the script it ignores it, and doesn't check it's return value. This is done for efficiency. In some cases the SMTP connection is complete before the VirusAction script completes. In this case the process is left in the defunct/zombie state when it finishes.
Don't worry about this however, the process will be cleaned up the next time a VirusAction is run. There should be at most one or two of these processes hanging around.
This occurs in with Linux kernels in the 2.2 series and earlier. Solutions include upgrading to a later Linux version, or ignoring the warning.