For Question 1, first write the set into $ \left\{ { a }_{ 1 },{ a }_{ 2 },{ a }_{ 3 },\cdots \right\} $, we can do so since it is countable. Then consider that for every $ \epsilon > 0 $, there is a interval $\left( { a }_{ i }-\frac { \epsilon }{ { 2 }^{ i } } , { a }_{ i }+\frac { \epsilon }{ { 2 }^{ i } } \right) $ containing $ {a}_{i} $ point. Sum them up and you will get the result.
For Question 2, if we assume A and B are measurable, by Cratheodory's Theorem, $ { m }^{ \ast }(A) = { m }^{ \ast }(A\cap B) - { m }^{ \ast }(A\setminus B) $, right hand side is equal to ${ m }^{ \ast }(A\cup B)-{ m }^{ \ast }(A\cap B)$. Meanwhile, without loss of generality, we assume $ { m }^{ \ast }(A) > { m }^{ \ast }(B) $. Then the left hand side is reduced to $ { m }^{ \ast }(A)-{ m }^{ \ast }(B)$. This is automatically true because ${ m }^{ \ast }(A)\le{ m }^{ \ast }(A\cup B)$ and ${ m }^{ \ast }(A\cap B) \le { m }^{ \ast }(B) $.