The phrasal verb "come off" is listed to mean to take place/happen not only in the Free Dictionary but also in other dictionaries (Oxford, Macmillan, Cambridge, etc.)
with examples similar to those mentioned by the PO.
Accordingly, I don't find any difference in meaning of "come off and take place"
except that (per these dictionaries) "come off" is informal and "take place" is common both in spoken and written English. Being a non-native speaker, I have no idea of how much the phrase come off is common or popular with native speakers.
So it's correct to say that "Is the meeting going to take place or come off today?"
Likewise, they are interchangeable in the examples mentioned in the question. However, I don't think it is proper to use come off for something that happens/take place by chance/accident or without being planned such as a car accident, a plane crash, etc.
Existing comments have clarified that it should be 'getting in touch with' or 'contacting'.
'Contacting with' doesn't work, though 'getting in contact with' is possible - I just wouldn't use it in either of the above structures.
I would also change the 'thanks'..
'getting in touch' is informal, so the structure
"Thanks for getting in touch with us"
works perfectly well.
On the other hand 'contacting us' is much more formal & I would prefer to see it book-ended with 'Thank you'
"Thank you for contacting us"
Best Answer
There is no difference in their meaning in the examples provided: wonderful opportunities are rare ... wonderful opportunities present themselves rarely...one does not happen upon wonderful opportunities often.
"Get" is by far the more common.
But the two locutions could have very different connotations in other circumstances.
How did you come by that fancy camera? versus Where did you get that fancy camera?
"How did you come by" could be understood to be an insinuation that the person with the camera had obtained it illicitly, whereas the question "get" is neutral; the questioner who uses "get" might well expect an answer such as "My uncle gave it to me as a graduation gift".