Do I have to be truthful if I want to submit a question in a newspaper? Can I fake that I’m, say, a grandmother, and make up an issue that totally doesn’t concern me?
If you think that the issue will be of interest to others, and that there is a legitimate question to be asked, it would not be forbidden to fake an identity for this purpose. Although the Torah commands us to distance ourselves from falsehood, if you are not causing anybody a loss by means of the lie (and, of course, the lie is not in beis din), and the only way to achieve a constructive purpose is by telling a lie, then it would not be prohibited. If there is no positive purpose to the question, then one be careful to refrain from telling a lie.
Sources: Many sources demonstrate that it is permitted to lie for various purposes. It is permitted to lie for the sake of mitzvos (for instance, telling family members that Shabbos comes in earlier than it really does, to make sure that the house is ready on time), and to avert losses (for instance, the Gemara teaches that one can lie to get out of paying charges that one is not legally obligated to pay). It would seem that the same principle would apply to other positive purposes, such as asking an important question in the newspaper, even when it is not related to the person asking. However, where there is no positive purpose, one must be careful not to lie.
The formal Torah prohibition of midvar sheker tirchak applies specifically to beis din, or, at least, to cases of potential loss for another.