Hi,
1. "Releases already effected are liable to be reset" for this error you might need to reset the release of the PO using ME29N.You can also check the status in EKKO field FRGZU (Release status). Try to reset the release of PO.
2. As you have used BAPI_TE_MEPOITEM, if you look into this structure it consists of an include structure CI_EKPODB (Customer fields for EKPO) which is also used in EKPO table.CI_EKPODB is used to add custom fields. Since you are using order_status fields, first of all the filed should be present in CI_EKPODB strucuture.Moreover there should not be any field with data type other tha Char.
3. You can also implement code related to custom fields in BADI ME_BAPI_PO_CUST using method MAP2I_EXTENSIONIN.
Hope it help.
Thanks