You can say 'come to Paris'. 'Come' expresses movement to someone's location. You can tell someone who's already in Paris that you will come to Paris, and they can tell you to come to Paris. However, once you are in Paris, you have arrived in Paris, as you are now located there. 'Arrive' expresses a change in location, so you 'arrive in Paris', whereas 'come' expresses journey to that location, so you 'come to' rather than 'come in'.

This depends on your precise meaning. It is correct if you mean you treated him kindly. However, if you mean you sympathise with him - a feeling rather than an action - it should be 'I was very sympathetic to him'. You can also say I felt sympathy for him.

Often they can both be used, especially in spoken English. However, in this case 'when' is more ambiguous without more context. If the sentence was 'this is not euthanasia, where a doctor gives a patient a lethal injection', it clearly means that if a doctor gives a pateient a lethal injection, then that is euthanasia. With 'when', though, it probably means the same thing BUT it could also mean 'when a doctor gives a patient an injection, it is not euthanasia (completely the opposite). To clarify, we would need to know a little more of what was said before and after.

It might be best to change it from passive to active voice: Despite the issue with the machine, we (or I/they/etc) found many cases of continued use.

关于节日的话，on表示节日在什么日子，比如Christmas Day is on the 25th December （圣诞节是12月25号）或Christmas is on Monday （圣诞节在周一）. At指“在什么节日的时候’’，比如'we will come home at Christmas'（我们圣诞节回家）。如果要表示在某个节日固定的时间，也用at，如 ON Christmas Day AT 11am （圣诞节早上11点钟）

The mening of both sentences is basically the same. However, 'entered' here sounds like you are describing an event carefully some time in the past, whereas 'came into' sounds more informal, and sooner after you actually came into the room.