Some misc C++ questions (multimap, derived class function argument, virtual static)

I have several C++ questions:

1. Why are the interfaces for multiset and multimap so complicated for
getting multiple values?

Instead of
multimap<Key, Value>

I usually prefer
map<Key, vector<Value> >

Isn't that much easier to use?

2. Suppose I have a base class that looks like:

class Base
{
  public:
        virtual void combine(const Base &other, Base &result) = 0;
}

class Derived : public Base
{
  private:
        string _name;
  public:
        void combine(const Derived &other, Derived &other) {}
}

When I try to instantiate Derived, g++ is telling me that the
pure virtual method Base.combine() is not implemented.
Why does Derived.combine() above not override Base.combine()?

Instead, I have to do some casting, like:

  void combine(const Base &other, Base &result)
  {
        Derived &a = (Derived &)other;
        string combination = _name + a._name;

        ((Derived &)result)._name = combination;
  }

Is there a better approach?

3. For my abstract Base class, I want to specify a static "Factory"
method that produces a Base*:

class Base
{
   virtual static Base* createRandomInstance();
}

I want to force all my Derived classes to provide such a factory
method that produces a Derived *.

However, apparently I cannot have a virtual static method
in C++ (or in Java). What is the best way to force that all
derived classes have such a factory method?