UML Sequence Diagrams – Showing User and User Interface Interaction

from UML Basics

The sequence diagram is used primarily to show the interactions between objects in the sequential order that those interactions occur...

so yes, it is perfectly valid to represent the sequence step 'ask the user for something'; each arrow does not necessarily correspond to one specific message

In a sequence diagram, you model the interactions between lifelines. Each lifeline represent a different instance of a type (aka a different object).

So in principle you would represent IBase, Derived and Derived2 in the same sequence diagram, only if there were several different objets of these types. You would then have a distinct lifeline for each object. Their interactions would not depend on the inheritance between the types, but on the "messages" that are exchanged between the instances (e.g. function calls and returns).

You would therefore not show the interaction between a derived class and its base class in a sequence diagram, because both lifelines would in reality refer to the same object.