I'm surprised you can ask a question about three completely different game elements.
So, first things first.
For the "Rubrium Potion", the one Imouto requires only once in the entire game (in current versions 0.90), you need to have a piece of the Philosopher's Stone in your inventory and return home through the "Star Door". Everything in quotation marks can be found in the forum above, using those words as search keywords.
The potion only requires defeating the Red Beast once and entering the Star Door once.
Milk is required for the "Do Something" -> "Drink Milk" function. There's a whole script that needs to be executed to unlock the function itself.
And a very important note. If Imouto doesn't fall asleep dead at 10:01 PM, it means she's already drunk the potion. Also, if there's a "Wake Up" option in the morning, that's also a sign that the Rubrium potion script has already completed.
The letter from the knight means that all "information exchanges [Insights] have been completed" and he has given to you everything he could for the harassment functions while Imouto plays video games (on the current version 0.90).
As for the [Videotape], and about obtaining, and about using, and about [white hearts] - I already described a detailed guide, look for it in the thread.
Words in square quotes can also be used to search the topic, as well as even my nickname.