Can someone who wasn't good in his 10th, 12th & UG (academics & extracurricular) still have a chance to get into Stanford/Harvard for MBA?

In pure numbers, the Harvard University admissions are way more competitive than the University of Cambridge. 

At Harvard, the acceptance rate remains around 3% to 4% every year because of a large pool of highly qualified candidates. On the contrary, the Cambridge acceptance rate stands around 2

Yes, MBBS in Harvard is a dual degree programme which leads to an MD. So MBBS is technically MD at Harvard.

Unofficially, yes, McGill University, located in Montreal is widely known as the "Harvard of Canada".

It is the country's best institution of higher learning as evidenced by its QS 2027 rankings, where it ranks #1 in Canada and #30 in the world. Look at the pointers given below -

• McGill Univ

Indian students who want to study at Harvard must have a SAT score between 1550 and 1600. Harvard Uni is very competitive with an acceptance rate of 4.2%. 

Indian students should have a good academic record in Class 10 and Class 12. In the school education, the studets should have a 3.9+ GP

Yes, Indian students who want to take admission to Harvard have to submit SAT / ACT scores for UG courses admission. To apply at the UG level, students have to apply via Common App. An application fee of USD 90 (Around INR 8.5k) has to be paid too.