Medically undiagnosed condition: how to inform?
I think you should inform your thesis comittee about your current situation. Best case, they are understanding, worst case they will ignore it for PhD-related matters. It might help to get a sick note from an independent professional even if the diagnosis is difficult. 

As to your second question, I would suggest to not disclose any health issues in the hiring process for the same reasons as stated in your linked questions, unless the position is specifically advertised as 'disabled applicants preferred' which I believe is a thing in Germany, for example.

Regarding your third question, I cannot say in general whether discrimination is to be expected. Depending on your condition and country of future employment, there may however be laws in place to protect you from discrimination against your condition.
I can only offer limited advice on how to deal with discrimination otherwise, but coping strategies against pain may be of interest to you in case of chronic problems, and may be subsidized by (potentially employer-paid) health care.