mommamia –
Yes, you are correct. The reason we don’t use the 18 minutes is because one opinion in the rishonim is that there is an understanding that by lighting the candles* you accepted Shabbos upon yourself, and therefore you aren’t allowed to do melacha. There are rishonim who disagree with this and the truth is that the letter-of-the-law follows the lenient rishonim, but there is a well-established custom to be concerned for the other opinion, which is why, as you correctly noted, we will only rely on it in extenuating circumstances.
*I am simply going with the assumption that you light at 18 minutes. If you haven’t lit yet you can do melacha until shkia; I’m not sure why R.T implied otherwise.