No. You are only limited by the number of components your APM license supports.
No. Each application instance name must be unique throughout the APM system.
The component of interest may already have its critical property enabled, or the component of interest does not exist within the same list as the critical component group. For example, if the component was inherited rather than added as a unique component.
Only the Service Check (WMI) component displays both running and stopped Windows services within the dialog.
The component of interest currently belongs to a critical component group; either the component needs to be removed from said critical component group or the group needs to be deleted prior to deleting the component.