虚拟语气还是推测可能性用法？

Miss Waterford, torn between the aestheticism of her early youth, when she used to go to parties in sage green, holding a daffodil, and the flippancy of her maturer years, which tended to high heels and Paris frocks, wore a new hat. It put her in high spirits. I had never heard her more malicious about our common friends. Mrs. Jay, aware that impropriety is the soul of wit, made observations in tones hardly above a whisper that might well have tinged the snowy tablecloth with a rosy hue.

请教老师：

句末...that might well have tinged the snowy tablecloth with a rosy hue.

1)这里的that是表结果的引导词吗?

2) 这里的might have done 结构应该是表推测可能性吧？但是我不能理解这里为什么是表推测可能性？表推测可能性，首先要某件事在过去实际发生了，然后才能推测，如果仅仅是假设（实际没有发生），那就不应该是表推测。

所以，如果这里是表推测，我理解不了。难道是表虚拟语气吗？但是如果是虚拟语气，怎么理解这种虚拟的含义呢？