We are dealing with two things here (1) Plant Physiology (2) How you are addressing this - and this is a question of how science is done.

Perhaps the best answer is for you to consider what happened? Did you do an experiment "wrong". Why do you think so? Turn the question on your head . What did the PLANT do? What is that telling you? You could grow it in a clay pot & why do you think that would make a difference? (IT would not - the stem would just die because it could not make it out of the top of the pot if the latter was solid). But you could do that experiment and you should then be asking what have I learned. What is the plant telling me?

You could play with the experiment. You could illuminate the upside down pot from the bottom - this would be an experiment to see if the plant is responding to light. You could also use a clay pot WITH a hole in its botttom (a drain hole) and a second without - and illuminate from both below & above as two separate experiments. (again, testing the role of light).

Finally, for your own pleasure (responding to (4) You should take a well- sprouted plant that was grown rightside up & suspend it upside down. The stem will turn. If you grew three such plants, and took two & turned them up side down & then took the third & depotted it and photographed the distribution of the roots, you could then in a few weeks take other two & de pot them, think you will see that the roots will be distributed towards the "top" of the pot not the bottom.

Again, your problem is that your hypothesis was "I expected the plant grow down, and then curve up." I think you assumed light would dominate. Your hypothesis was disproven. You now need to further test light. And you need to erect another hypothesis (also tested here), that gravity dominates over light.

(Getting to the bottom of this gets into primary literature, e.g. the response is to auxin distribution, in turn probably (in my understanding) Having to do with some sensing of gravity by the plant - (see Gravity signal transduction in primary roots, Perrin et al 2005. (Annals of Botany Volume 96, Issue 5 Pp. 737-743) This suggests that columella cells of the root cap sense gravity & then influence the redistribution of auxin, in turn determining the growth of the root - in this case downwards.