1) Example of Ilm al Ijmali:
A person may know that impurity has occurred in one of the vessels, but he does not know precisely whether it is in the first, second or third vessel.
Similarly, a person knows that the most knowledgeable exists but does not know exactly who is the most knowledgeable, therefore he can follow any one of them about whom we have Ilm al Ijmali and not the rest.
2) No it is not the same as Ihtiyaat as in that case you need to learn views of all the mujtahids irrespective of whether they are aalam or not.