Answer 1

If my understanding of your diamond problem is correct, then the problem in a such a case is that for e.g:
There are four classes; A, B, C & D
B & C inherit from A
D inherits from B & C
The problem here would be that D would be in a sense inheriting from class A twice; once from B and once from C.
This would cause a problem since this would not only take up more space, but also when D calls some data member or method/function from A, the compiler would not know which one it should access.
In such cases, A would be inherited virtually to ensure only one copy of A is inherited by D.