1. Yes, it is possible to attach Excel file(s) and it can be opened before any approval step within workflow.
2. It is possible to maintain Long text, not limited to any number of words.
3. No, standard system will not popup anything, but specific approvers can be validated with ABAP exit when AR is saved.
4. Approvers are identified as SAP users and not as HR mini-master records. Not every HR record is created as SAP user, and standard workflow operates in terms of SAP users. Anyway the mapping is normally maintained in the Info Type 0105 (Communication) SubType 0001.