Very nice. And quite difficult too
About ことの方が, I think, just like another ~方が, it means "than" (comparison function). Like in A is .... than B. Some times B can be hidden like the above sentence.
延々 function is to emphasize the 持続する, so it means keep continue without any interruption. I like this phrase type. another example, ゆらゆらと歪んだ空.
Ehm, not sure about 思われる、~ように思われる. I think, it shows people opinion in general.
due to my english, I can't help with the translation. But I hope add some phrase can help you understand the meaning.
For example,
(全く知らない人と仲良くやることよりも)
かつては熱々だった相手と冷め果てた関係を延々と持続 することの方が、寒々しく耐え難いように思われた。
or
かつては熱々だった相手と
(そのまま別れるよりも)冷め果てた関係を延々と持続 することの方が、寒々しく耐え難いように思われた。
Ehm, the second interpretation makes more sense.
lets wait Native comments.