each grade is assigned a point.

ex. A = 4.0, A- = 3.7, B+ = 3.3, B = 3.0....etc.

A+ is still considered as 4.0, the only thing that sets it apart from A is the marks it represents and it is typically awarded to students who achieve 90 or above...

No, getting an A+ or A won't affect your cgpa since gradepoints-wise they are the same (4.0), they differ only by the marks they represent, but getting an A- will definitely affect your cgpa since it carries a grade point of 3.67 or 3.7...

Nope, the said rule only applies only to A+ and A, all other grades after that have their own pointers, with B+ = 3.33, B = 3.00, B- = 2.67, C+ = 2.33, C = 2.00, the rest is a bit of gray area because while some universities assign pointers to grades below C, some consider them as fail and assign a pointer of zero...