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).