All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we should say that when the scholars spoke on the issue of traveling to non-Muslim countries and were strict on this matter and forbade traveling to these countries, it is because of fear of temptation in religion.
Those territories, where the majority are Muslims and belong to a non-Muslim country; we do not know if one is safe there regarding temptation in religion.
If we presume that the Muslim is safe in his religion there, then we hope that there is no sin in traveling to it for tourism or anything else. This is regarding your first question.
As for the second question, we should not be heedless of the Hadeeth reported by Abu Daawood and At-Tirmithi on the authority of Jareer ibn Abdullah that the Prophet said: “I am free from (i.e., disown) any Muslim who lives among the polytheists.”
Moreover, the presence of the Muslim in a country where the Muslims are the majority, the Athaan can be heard, the obligatory prayers are held in mosques and other rituals, is better for him than going to a country where these rituals are not seen except seldom, and where people openly disbelieve in Allah.
Yes, there may be certain cases in which a Muslim is forced to go to a non-Muslim country and his presence there is better in regard to protecting his religion and distancing himself from harm, because if he stays in his country, he is tortured and may be killed. This is exceptional verdict certainly does not apply to the one who wants to travel for tourism only; so you should be heedful of this.
With regard to your third question, the answer is that migration from the land of Kufr to Muslim countries about which the early scholars spoke is the example that you mentioned in the question, which is the migration of the people who embraced Islam in their non-Muslim lands to the land of Islam. So, if any person from these countries becomes Muslim and he is unable to establish the rituals of religion, or fears temptation in his religion, then he is obliged to migrate if he is able to do so.
However, if he is able to openly practice his religion and does not fear temptation, then migration in his case is merely recommended and not obligatory. This is because by not migrating he would increase the number of non-Muslims, and thus migration saves him from mixing with them and seeing the evil they commit amongst them.
For more benefit, please refer to Fataawa 369667, 1818, and 89879.
Allah knows best.