Muharram & Aashuraa

Fasting on only the 10th of Muharram due to some valid excuse

Q: I have read that it is makrooh tanzihi for one to fast on only the 10th of Muharram (without also fasting on the 9th or 11th). My questions are:

1. What is the ruling if one could only fast on the 10th due to some valid reason?

2. Should he rather not fast on the 10th?

3. What constitutes a valid reason?

4. What's the meaning and ruling of makrooh tanzihi?

Fasting only on the 10th of Muharram

Q: Some people say that it is not makrooh to fast only on Ashura, the tenth of Muharram, because it is no longer the practice of the jews in today's times. Rasulullah (Sallallahu Alayhi Wasallam) mentioned in the Hadith that we should not only fast on the tenth as we will be resembling the jews. Since the jews no longer fast on the tenth of Muharram, it will no longer be makrooh for Muslims to only fast on the tenth. Furthermore, they cite the following quotation of Badaai'-us-Sanaai' in support of this claim. Is this view correct? If not, then what is the correct explanation of the following quotation?

وكره بعضهم صوم يوم عاشوراء وحده لمكان التشبه باليهود، ولم يكرهه عامتهم