Scholars

The historian Muhammad Jarir al-Tabari was Sunni. 

Ibn Shihab al-Zuhri: While Shi'ism originated early (a Shi'i being someone who supported the leadership of Imam 'Ali (A) and his descendants), it took some time for Sunnism as we know it today to develop. Al-Zuhri lived somewhat before that time. So it is not always wholly accurate to identify people in his time period as "Sunnis". This is not an issue with Tabari since Tabari lived later. 

In any case, Ibn Shihab al-Zuhri's work on hadith was the beginning of what became the tradition of major hadith collections in Sunnism, and led to the formative Sunni books of hadith. Furthermore, he was associated with the Umayyads. Therefore, he is a key figure in the development of Sunnism. 

It is good to be a scholar, and it is good to be a doctor. Society needs people do to all the different jobs, or else it wouldn't function. Ideally, it is good for you to do the job that you are most talented at and most committed to.

It is narrated that the Prophet (S) said: ‘Knowledge is of two categories:
knowledge of religions and knowledge of the physical body.’ So this narration celebrates and values both kinds of knowledge (religious and medical).

(Of course there are other important jobs and fields of study as well!)

As you know, there are many narrations from the Prophet (S) and Ahl al-Bayt (A) about various medical ailments, so it was clearly a subject that was important to them. So inshallah you are also following in their footsteps by being a doctor. 

Although belief in tahrif of the Qur'an is non-standard, it is better to reserve the word kufr for the things that Allah directly uses it for.

The Qur'an does not directly say that belief in tahrif is kufr. (Rather, some people assert this based on a deduction/interpretation of a verse, not a direct statement.)

The word kufr tends to be thrown around quite casually these days. There is rarely any benefit to accusing people of kufr or labelling Muslims as kafir just because one thinks they are incorrect in their belief. It is quite sad that this intolerance has developed today. The classical Islamic era seems to have been much more tolerant and open to discussing differences of opinion regarding theology. This is not to say that every view was always right, but it is better to be able to discuss things rationally rather than to shut down dissenting views by just labelling and attacking (verbally or sometimes physically).

Some scholars have rejected the idea of the theory of evolution as being against the Qur'anic teaching of the creation of the human being, whereas others have accepted it as being compatible with the Qur'an and as being Allah's plan for the human being and how to engage in creation.

It seems that there is some generational factor here - the older generation tended to reject it more, perhaps because they saw the theory of evolution as being associated with colonialism or secularization, and as an attack on traditional Muslim values. This is less of a factor in today's globalized world. 

At the same time, the theory of evolution is only a theory and cannot necessarily be said to be true either. It is simply considered an acceptble possibility, pending further evidence, by some scholars. 

The idea that life originated from water is supported by the Qur'an. 

One could somewhat nebulously suggest that the idea of "nasnas", or prior types of humanoids, which appears in hadith, could also support the idea of evolution, although in my view this may be a stretch in interpreting the hadith. 

There is a paper on Shi'i scholars' responses to evolution in the conference proceedings for this conference, if you are interested in reading it. https://www.islamic-college.ac.uk/publications/shiistudies/sixth-shii-co...

There have been great Islamic scholars throughout the Islamic world, including regions which were part of the Persian Empire and the Roman Empire.

Scholars who are from regions that were once part of the Roman Empire include scholars from Al-Andalus, North Africa, parts of the Arabian Peninsula, and some other parts of today's Arabic-speaking world.  (I'm sure you can find some examples from each region if you look!)

That said, the centre of the Roman Empire (Rome) and the regions immediately surrounding it (such as most of Italy and Greece) never became part of the Arab-Muslim Empire. Usually the capital or centre of a civilisation has the greatest scholarly output and resources. (That is apart from the conquest of Byzantium. and that happened rather late in Islamic history.) In contrast, the heart of the Persian Empire was absorbed into the Arab-Muslim Empire. So this could be a factor.

It could also be a matter of nomenclature. The regions of the Roman Empire that were integrated into the Arab-Muslim Empire were usually more on the fringes or outskirts of the Empire, and generally the people there did not identify themselves as "Romans"; they were simply under Roman control. For instance, Islamic scholars from Egypt would not have identified themselves as "Roman" despite the fact that Egypt was once a Roman province. In contrast, many people from the Persian Empire were identified as "Persian".

So, perhaps these may be factors.

In any case, there have been Islamic scholars from all these regions.