> Q1. I wonder if these two expressions are more or less
> interchangeable?

Yes, they are. They mean the same thing : If the statement happened, you wouldn't mind. Even more, you regret it did not happened.

> Q2. Are they both considered litotes, making them sound as if I'd
> really have liked to take a break?

In my own opinion, yes, they are. Because of their negative construction. (But I'm not a french professor. Even if I'm french, I'm not expert in figure of speech)

> Q3. Is it acceptable to use the constructions "à + infinitif" and
> "contre + infinitif"? Or should they take the form of "au fait de +
> infinitif" and "contre le fait de + infinitif"?

Yes, it's acceptable.

**Little french tip :** after "de" or "à", their will always be an "infinitf" (if it's a verb).