Regarding your first question: if we were talking normal probability, it would be like being told X ~ B(4, 1/3) and saying
p(X=k)=811(k4)24−k instead of
p(X=k)=(k4)(31)k(32)4−k.
The first expression isn't
incorrect, but it's probably not the right thing to do here, particularly if you're trying to relate the expressions you get to binomial probabilities.
As for your second question, possibly knowing the pgf for a binomial would help you. Or if you think about it, the coefficient of t^k (q + pt)^n is the "probability of getting k ts" out of n trials where you have probability q of choosing the constant term in a branch and probability p of choosing the t.
At the end of the day, this is very much about being familiar with binomial expansions and pgfs; if you are, the link is obvious (in fact, you can deduce that Y is binomially distributed purely from considering pgfs if you know what you're doing).
I think at this point your best bet is to move on and try some more problems and try and improve your intuition.