R – how to delete entry in content database while delete list item

You will need to use a List Event Receiver and tie into the ItemAdding event - http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spitemeventreceiver.itemadding.aspx

There's a good tutorial here: http://msdn.microsoft.com/en-us/library/bb736146.aspx

And a good little tool for attaching the event receiver - http://lirliron.blogspot.com/2007/08/moss-list-event-receiver-register_08.html

You can do the check against the To and From and see if there is already an item with that in the list, and if there is set the SPItemEventProperties.Cancel appropriately.

Generally, the scope of lists is the SPWeb that hosts it. That means that you cannot reference a list by id from a subweb. You can work around this by writing a control that opens the list's parent web, and then opens the list. There may be other more elegant solutions to your problem though.