She calls her boss by his first name which is the same as her husband's, but she calls him Mr in every other episode... what is that all about?

Since he was calling her during non-working hours she probably felt more casual, and dropped the formalities.

Maybe. But she always referred to him as Mr, even at hone, when discussing her job with Doug or her friends.

When Leah says I love you to her boss .....

Leah didn't say I love you to her boss. "Carrie Heffernan" did.

And yes, it is kind of odd since Carrie always called her boss "Mr. Pruzan" every other time. That episode was the one and only time she called him Doug, but maybe it's because of the "I love you" mistake that she never called him Doug again.