I would like to share few points based on my knowledge, not sure all those will be fully correct.
- If eNodeBs with X2 link not connected to same MME, X2 handover can't be performed. However, both can exchange other information.
You have to go through S10 based handover and make sure first MME (source MME in case of handover) includes the list of equivalent PLMN to UE in Attach Accept. Equivalent PLMN list should contain PLMN of target MME.
In case handover fails at target cell, UE perform the cell re-selection and it may select the same target cell which was prepared for the UE as part of handover signaling. Since target cell is already prepared, re-establishment for UE at target cell will be successful.
UE will perform TAU with target MME because current TAI of target cell won't be matching with the TAI list it got from first MME.
Since current serving PLMN has been changed for UE and PLMN is used to derive Kasme at HSS, I think target MME should re-perform EPS AKA and Security Procedure. I am not sure, target MME NAS can continue with the same Kasme (inside MM Context) sent by source MME in Forward Relocation Request message, let others to respond on that part.