He came here alone. (Fine)
He came alone here. (If not incorrect, sounds unnatural)
Maybe one day I can visit you.
Maybe I can visit you one day.
One day, maybe I can visit you.
These are all fine and sound natural.
He says hello to me.
Means you generally greet someone, doesn't mean you say 'hello' specifically.
He says to me, "hello".
The structure means it is reported speech, and thus should be in quotation marks.
I agree with Dan Smith about the tense usage by the way. As stand alone sentences they should be in the past tense.
If you are describing something that happens in a routine you could use the present tense e.g. 'He says hello to me every day on the way to work.'