Does writing (not fiction) something untrue in itself, even if not
shown or deleted, cause one to have uttered a lie?
Writing fiction and non-fiction text is not bad if your intention for writing is pure and with accordance to Buddhist ethics and precepts. Example, if your end goal isn't deception of the audience or personal gain, or lust - writing is fine. There is Buddhist art, poetry, stories and many architectonically beautiful temples which are also pieces of art. You shouldn't get carried away by signs in Buddhism, but you should also use them skilfully. There is the Chinese phrase of seeing the mountain, not seeing the mountain and then seeing the mountain again.
There are statues of Buddha and paintings that arouse great joy when needed, and that is definitely not negative. Ajahn Sumedho in his Four Noble Truths wrote that we shouldn't be those nagging old people that deny woman's beauty because its the fruit of conceptual mind - instead, we should marvel in women's beauty, there is vast ocean of multitude of shapes and that is joy.
Does speculation about something, where ideas are asserted without
certainty, consist in lying as well?
If you presume that there will be no rain today, your are not doing anything strictly negative. So, speculation is not per se negative, but it is not the idea of Buddhism. Lack of complete knowledge is not lying, it is unintentional. There is in fact, numerous Buddhists among Physics community. All we do in Buddhism in the end is spiritual pursuit born of not knowing and wanting to find out. We do not know how it is at the end of the road, we can only guess. On absolute truth, Buddha said that any speculation of finding the absolute is not within the scope of Buddhism. Also, Buddhism doesn't have a take on standards and political organisation of society either, or crime and punishment in society, etc. Such speculations and ideas are beyond Buddhist's scope, though many Buddhists embrace feminism, vegetarianism - and many not.