HI @aditya_pathak,
a) Yes a single ServiceNow Service Desk application can be used for creating incidents for multiple IdentityIQ applications.
You can also configure multiple ServiceNow Service Desk applications into the IdentityIQ, where each can cater to specifically one or distinct groups of IIQ applications, as per organizational need.
b) If you have configured multiple applications under tag for Incident ticket type, which will create only a single Incident. So these fields which are referenced one can have only one value, but for string type fields like Description can have multi-line info.
For customizing planinitializer rule you need to get in touch with SailPoint Services / Expert Services team for doing it.
c) Yes the list of applications that are mentioned under tag in the specific IdentityIQforServiceNowServiceDesk application instance will be served by that particular instance of Service Desk application (ServiceNow SDIM).