Reflecting the broad range of issues that have come to characterize relations between the two countries, some of the sensitive topics that have dominated U.S.-Chinese talks in the past now seem further down the agenda. 

1. 红色部分是本句的主干。两个蓝色部分都是定语从句。绿色部分是现在分词作状语，其逻辑主语是 some of the sensitive topics。

2. some of the sensitive topics：主语。now seem further down the agenda：谓语。

3. 蓝色部分that have come to characterize relations between the two countries 是 issues 的定语从句。蓝色部分that have dominated U.S.-Chinese talks in the past是some of the sensitive topics 的定语从句。

【翻译】过去主导美中会谈的一些敏感话题，反映出赋予两国关系特色的广泛存在的问题，现在似乎又重新落到了议事日程上。

就句论句而言，

1. 把Reflecting...还原回去, Though they （= some of ...topics）reflect才会靠谱

2. have come to characterize 业已形成特色。。。

3. have dominated 实际是  一直以来...

4. down the agenda, 推迟；不再在什么会谈中具有优先权

大意：

一直鬼吵的，现在可以吵个毛了

Reflecting the broad range of issues that have come to characterize relations between the two countries, some of the sensitive topics that have dominated U.S.-Chinese talks in the past now seem further down the agenda.

参考译文：反映了两国之间的关系特点的十分广泛问题，一些过去主导美中会谈的敏感话题现在似乎在议程上进一步后置了

hat have come to characterize relations between the two countries为定语从句修饰名词issues

reflecting 的逻辑主语是后面句子主语