The first trip which was expected to take place in the middle of this year was pushed back to at least 2021

because of the coronavirus pandemic and the new safety measures it requires.

这是CNN新闻播报员说的。

1)  句子开头处trip后的which是否应该改成that？因为先行词有first限定。对吗？

2）句末处，定语从句the new safety measures it requires. - 如果it是定语从句的主语，那么it指代什么呢？ 如果it不是主语，是否应该去掉it,改成关系代词which或that作主语，同时require应该是不及物动词，

     那么究竟应该怎么样理解分析呢？