Is IPsec IND-CCA secure provided the used block cipher is a pseudorandom function?
I would like to know which mode of operation is used by IPsec 


That depends on what is negotiated; if we assume we're using AES as the block cipher, then the standard modes (that is, ones which have transform numbers assigned by IANA) are CBC, CTR and GCM.


  and whether my approach makes sense.


Sounds good to me.