Is there a substantial difference 断【ことわ】る and 真【ま】っ平【ぴら】?
Yes. For starters, 断【ことわ】る is a verb, and 真【ま】っ平【ぴら】 is a -na adjective.
Can you replace one with the other without affecting the meaning?
- 断【ことわ】る means "to refuse, to reject, to turn down a request".
- 真【ま】っ平【ぴら】 literally means "completely flat, completely open; entirely [so]".
Idiomatically, the meaning has shifted in general usage, and now the term is more commonly used a bit like "flatly [no] → absolutely [not]". Historically, the word was used so often in a kind of fixed construction with negatives that it became shorthand for the negative sense, and the actual negative word is often omitted anymore. Think of it like "completely unacceptable, absolutely not".
Note that 真【ま】っ平【ぴら】 is a very strong statement, imparting an unambiguously negative value judgement, often with overtones of intense dislike. 断【ことわ】る, meanwhile, is much more neutral.
Here's an example. Let's say there are two fellows talking about a woman interested in one of them. She's asked the one fellow if they could go see a movie.
- If the askee says to his friend 断【ことわ】る, that just means that he'll turn her down, ostensibly in reference to this one time. Maybe he has a doctor's appointment, maybe he's sick, maybe he's not sure about her, we don't know. They might already be dating, or even if not, they might still do something together some other time.
- If the askee says 真【ま】っ平【ぴら】 instead, we know that he's completely not interested in her -- even more than that, he is actively disinterested in her, and will likely never date her.
Please comment with any questions about the above.