you only need one menuitem to show your leads (you can create from within it).
so first the action:
<record id="show_lead" model="ir.actions.act_window">
<field name="name">Lead</field>
<field name="type">ir.actions.act_window</field>
<field name="res_model">crm.lead</field>
<field name="view_type">form</field>
<field name="view_mode">form,tree</field>
<field name="view_id" ref="crm_case_tree_view_oppor_extended"/>
</record>
and the menuitem:
<menuitem name="Show Lead" id="show_leads" parent="sub_menu_lead" action="show_lead"/>
just delete the create_lead action and menuitem
and a little hint: if you got xml errors, go to the stacktrace riiiiight before the output of the xml part (the record where the error belongs to) and you will mostly find the real error message like "can't find field anyfield in model anymodel"