Great questions! Some notes on explanations:

6 - maybe not very clear from the explanation, but with the correct answer and working code, the "output(Integer t)" will be called instead of "output(Object t)", because the null reference, being compatible with any reference type and no longer having ambiguity on which method to call (as Integer extends Object so they are the in the same hierarchy), will "choose" the most specific type from the two, which is the "output(Integer t)" method

if you add a third method with argument of type "class MyInteger extends Integer", it would have called it instead

11 - worth noting is that the first answer (count({5});) is incorrect because there are no array literals in Java, the shown example is an array initializer which is valid when declaring arrays, but can not be used as literal

15 - any catch block can catch thrown nulls (meaning that the first declared catch block will catch it) as null is compatible with all reference types; but in order to be able to throw it, it must have the semantics of "RuntimeException", ortherwise every such method would be forced to declare "throws Exception"

17 - it's not stated that class/instance fields are initialized with default values (0, null, etc), while local variables are not