A-they.We do not know what the antecedent of they is.If the comment is more important than the people who deliver it, a passive form sounds better.
B-his.We cannot be certain about whose vacation the sentence is about.A reordering of the sentence would be right such as.After Phil´s vacation he visited Uncle James or After Uncle James´vacation Phil visited him.
D-We don´t understand who had the accident.It could be written like this. After Lian´s accident Julie helped Lian to fix her broken down bike