Re: in which, to which, at which..all about which..
Take these two sentences:
This is the house. Jack built it.
Now, if we wrote English like that all the time, it would sound like a book for very young children who are learning to read. We can combine these sentences using a relative clause:
This is the which Jack built.
OK, that's easy enough: but how do we combine these two sentences?
This is the house. Jack lives in it.
We can't simply say: "This is the house which Jack lives." It doesn't make sense -- we need to fit in the sense of "in". That little preposition mustn't be allowed to disappear.
There are two ways of doing this: the traditionalist and the modern. The modern way is just to leave "in" at the end of the sentence, like this:
This is the house which Jack lives in.
We don't need "it" any more, because that is included in the relative pronoun "which". But many people object that putting a preposition at the end of a sentence like this is wrong, so for formal English you're better off using the traditionalist approach, which is to put the preposition in front of the relative pronoun, like this:
This is the house in which Jack lives.
So no, "in which" is not an equivalent to "which". You can do the same with many other prepositions, e.g.:
This is the school. Jack goes to it.
-> This is the school to which Jack goes.
This is the disco. Jack met his girlfriend at it.
-> This is the disco at which Jack met his girlfriend.
This is the house. There is a long nursery-rhyme about it.
-> This is the house about which there is a long nursery-rhyme.