Reb Shlomo Zalman Auerbach allowed one to change the setting on a Shabbos clock to keep an appliance that is currently off from turning on, because it does not cause a fire to be extinguished just never to light. However many Poskim disagree. Additionally, Reb Moshe Feinstein was concerned that moving the pins is muktza. So while there are grounds to be lenient, it is advisable not to.

However a couple of side points. Where to you keep your cholent overnight if the platta is off? Also, what are you putting on the platta when it comes on in the morning and how? Wet, cold foods may not be placed on the platta at all, and even dry foods may only be placed on an intervening pan according to most opinions.

There is reason in Halacha to cover the head for kiddush out of respect for the beracha, however a hat would certainly suffice.

Halacha does not require one to wear a talis for kiddush, but some Chassidim have such a minhag. The is a Pesikta that writes one should fulfil "zachor es Yom HaShabbos l'kadsho" through "atifa" which usually refers to the talis. This pasuk is the same one used to derive the obligation for kiddush. Therefore the Minchas Eluzur of Munkatch wore his talis for kiddush in the morning, but the Rebbes of Satmar and Sanz did not. Since this is an issue of minhag, one should follow the custom of his father.

First of all it should be clear that Adderall is a Schedule II drug under the Controlled Substance Act and is a violation of Federal law to take it without a prescription from a physician. Amphetamines can have serious side effects and lead to addiction, and the Torah does not expect you to significantly risk your health for the sake of learning. I will assume you are taking this potent drug under the consultation of a qualified and reliable doctor.

Regarding Hilchos Shabbos; while generally medicines may not be taken on Shabbos, however this only applies when they are treating an ailment. A pill that is being used to enhance performance is not included in the issur and therefore it seems to be mutar similar to a caffeine pill. However, since Adderall is primarily used to treat ADHD, which may be considered an illness, it is not so clear. An additional lenient factor in your case would be that some Poskim permit taking any medicine for a mitzva, although most do not. Ideally the extended release form of Adderall could be taken before Shabbos begins, and there would be no issues of Hilchos Shabbos.

It is not necessary to finish the entire meal before shkiya, but if one eats bread after sunset there is a dilemma whether to say Retzai or Ya'aleh v'Yavo, so it best to at least finish the challa before shkiya.

Let's start with the practical issues, the main practical difference between a blech and a platta is that the platta is a single uniform temperature over the entire surface while a blech can have hotter and cooler spots. So a platta is more predictable, but a blech gives you more options. Regarding the cost, I don't think the difference is significant, and in any event Shabbos expenses are refunded by Hashem

Regarding the Halacha issues, raw food may certainly not be cooked on Shabbos, whether on a bleach or a platta. Food may be placed on either before Shabbos begins, because the only issue is that we are concerned if one placed raw food on a fire shortly before Shabbos that he may become impatient and stoke the flame on Shabbos. With an electric platta that has only a single, unadjustable heat setting this is not an issue, and a blech is considered an unusual manner of preparing food that would remind one not to adjust the heat. The remaining issue is, as you mentioned, reheating cold, dry precooked foods on Shabbos, where actual cooking does not apply since it is already cooked, and we have already determined that stoking the fire is also not an issue with either a platta or a blech. However there is an additional prohibition of doing an action that appears to be cooking. Since a platta is designed and suitable only for reheating and not for cooking, some Poskim permit placing cold dry precooked foods directly on a platta and consider it more lenient then a blech which is on the stovetop, which is the normal location for cooking. According to the more stringent opinion a platta is identical to a blech, but there is still a solution to place the food you want to reheat on top of an upturned empty pan. This avoids any problem of appearing to cook. It must be emphasized and made completely clear that this only helps if the food is both dry and completely cooked before Shabbos.

The accepted Halacha is that the key must serve a functional purpose to be permitted and not just be attached or even decorative. I have seen a belt buckle where the key is attached in place of the prong that is inserted in the hole in the leather belt, and this is ideal.

A Rav would have to be very big or have very clear sources if he wants to permit something Reb Moshe forbade and none of his contemporaries disagreed. So it is unlikely, but it is possible.

There is no problem leaving the food in the oven if it is half cooked and edible, however when you open the oven door to remove the food there is a strong likelihood that the thermostat will immediately ignite the oven. Ideally the timer should be set to turn off the oven shortly before you anticipate wanting to remove the food on Shabbos. If this is not practical, you should only open the oven door when the fire is already on, and you will at worst be indirectly causing it to remain lit for longer but not igniting it on Shabbos.

It is permitted because one may assume that the person one is buying from is from the majority of the population, which in the US is not Jewish.

It's a problem because you have no use for the talis at home for the rest of Shabbos, so in effect you are preparing for Sunday which is forbidden on Shabbos.