When I turn a sentence into reported speech, for example 'I work here' it becomes 'She said she worked here'. But what if I start the reported sentence with 'She says/ she's saying' would I still write 'she worked here'? Or would it be: 'She says/is saying she works here? And if I start with 'She had said' or 'She's said' or 'She's been saying'. What tense should I use for the verb 'work'?
'She said she worked there'. 'She is saying she works there' 'She had said she worked there'
Those are all idiomatic and I think you already understand them.
But when it comes to 'She has said' and 'She has been saying' you need to go back to the present tense, because 'she has said' does not relate to a specific past event. So you need:
'She has said he works there' and 'She has been saying she works there'.
You may find the English Language Learners site helpful.