When reviewing the を境に grammar point, it feels like the good answer is 50/50 between を境に and を境にして with no way to discriminate between the two options. There is no explanation anywhere why one is correct in a given situation and the other is not, which feels punishing in a very arbitrary way.
Just at a glance: を境にして seems to occur right before a comma. As I haven’t done the grammar point myself yet, it’s just a wild guess. Could someone confirm? 
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I found a resource online that says they carry the same meaning and can be used interchangeably. Source: Hanabira
This one has the explanation in Japanese and says a similar thing as Hanabira: また、「〜を境にして」は「〜を境に」にチェンジすることができます。
Source: 〜を境にして・〜を境に｜日本語能力試験 JLPT N１文法 : 絵でわかる日本語
So I believe you’re correct about no way to discriminate, haha!