GRE

To go to abroad, most countries accept LETS score. You should go for offline learning as physical interaction is must. Apply for them 3 months before your admissions get started.

https://studyabroad.shiksha.com/usa/universities/columbia-university
Follow the link to get detailed information about Columbia University.

All the universities abroad accept GMAT score for getting admission in MBA programmes.

Yes, GRE is not mandatory at all in Canada. It will just enhance your application of admission. IELTS is very important and score at least 6.5 bands with 6 minimum in each component.

Yes, if you have a very good score, then they might give you a scholarship but not 100%.

https://studyabroad.shiksha.com/exams/gre
Use this link to know about GRE.

Pass score depends on the college you apply for. Generally, 310+ out of 340 is considered a good score in GRE. In TOEFL, 80+ out of 120 is the minimum eligibility criteria of English proficiency. In GRE. The verbal section is very hard, so you need to prepare well for that. TOEFL is easy, just a month preparation is enough to score well.