Observing purdah from one's daughter in law's father

Q: My dad is late (may the Almighty grant him jannat). He has left behind 2 sons and 2 daughters. The eldest son lives with his father in law.

After mummy's iddat, it was her desire to perform umrah and her second son took her Alhamdulillah. The elder son wants to take my mother, his wife and his father in law, who is also a widower, for umrah. Will it be permissable for my mother to travel like this in the company of his father in law?

Manager creating a position for a person he likes in the company

Q: If a manager creates a position to do some work in a company (in which he is employed) when he knows that there is no need for such work, and appoints a person he likes or favours on this position, and the appointed person knows all this (what the manager has done), will the appointed persons salary be halal or haram although he is doing what is stipulated in his contract?

Taking prophetic or non prophetic medicines

Q: Should we only rely on spritual medicines like zam zam, honey, etc. and not take medicines? I don't know why I think due to my weak faith that what happened if I don't get shifa and I only think of using medical medicines. Am I committing kufr?

I heard the Prophet (sallallahu alaihi wasallam) used both. Did he always use then in a combined form of both spiritual and physical medicines?

Celebrating birthdays

Q: If someone follows the opinion that celebrating birthdays is halaal as long as they are not extravagant and as long as they don't include something haraam, then if that person says happy birthday to non-Muslims who are celebrating it, are they still Muslim?

Because I've heard from people that if a Muslim participates in a kaafirs events then they have committed kufr (atleats that is what I understood) but what if that person thinks its ok to celebrate birthdays?