relations : Equivalent classes axioms, subClassOf axioms

Hi,Please, explain me these ambiguous points:
   
   - If we already have C1 subClassOf  C2 , Why cannot we add C2 subClassOf C1? (It is mentioned in some works but for which reasons?)   
    - Is it possible to axiom to add this axiom , delete the two subClass axioms and transform them to equivalentClasses axiom(C1, C2)
   - Is equivalent class axiom (C1 , C2) entails that   C1 subClassOf  C2 and C2 subClassOf C2. (please I want to know also this case where C1 and C2  can be arbitray class expressions? ex C1 subClassOf (allValuesFrom (R, C4))
   - For protégé, when I tried these cases, given C1 subClassOf C2  and  adding C2 subClassOf C1  => It entails that equiavlenttClassesAxiom(C1, C2) , we have no C1 subClassOf C2 and keeps C2 subClassOf C1
Please help me in finding explanations for these thoughts.