> I am guessing that the long form adjectives 'хорошая' and 'грустная' are here instead of 'хороша' and 'грустна' because the latter would sound a bit bookish in conversation?
In this case "хороша" is nothing unusual. "хорошая" is more universal word. For example, you cannot say "я купил хорошую книгу" using short form. So, "хороша" is naturally more rare.

> Also, 'плакала' (imperfective) is used to explain that 'I was crying when...' rather than just 'I cried when...', to emphasise the crying and make it a bit more possible to empathise with?
Em... "я заплакала" = "I had started to cry when...". What russian word do you mean behind 'I cried when'?