1. Delivered e-mail already stored in the SME user folder unless forwarded to another account outside of SME. Therefore e-mail that already received, will stay, but the new e-mail to the removed virtual domain will not be accepted. Proxypass only affects HTTP, HTTPS redirection, not the e-mail, ssh or other TCPIP services.
2. Yes. Using IP number instead of DNS name works, as long as that IP is accessible from the SME server. ProxyPass uses full exact URL, so you can differentiate between xyz.virtualdomain.com and abc.virtualdomain.com. using just virtualdomain.com (without abc. or xyz.) will not be resolved unless proxypass redirector is also used for it.
You also can use
http://klm.virtualdomain.com ->
http://1.2.3.4/klm/ format to use subfolders on the webserver