A1.

Consider following example

class A { method XYZ(//some implementation)}

class B { method XYZ(//some implementation)}

class C extends A,B

Now if you say C c = new C() and C.XYZ() java don't undestand which implementation to use.

A2.

Inheritance achieves structure, reusable code

A3.

main() method is required to start new java process. In servlet,JSP (which are just class not process) container is responsible for life cycle of both. Container itself is part of some running process.