Aagh! Frequencies! MTed will likely be able to do a good job of answerin gthis thing. I will answer the first part of the question though.

Yes a tune written in D will have all the notes one tone higher than the same tune written in C. Why bother with keys? They are a structure that the music is built in and around. Think of it like an apartment building. Every apartment has the same layout but they are on different floors. The floors stop you from losing your way and falling into other people's apartments. You can walk into any apartment and turn right to open the closet door. Troglodytes with a fear of heights live in the basement and claustrophobic birds live on the top floors.

Different keys can be played more easily than others on instruments and when you put different instruments together you need to find a suitable key. You're right when you say that chords with open strings sound different from chords with no open strings, they do and the guitarist will want to play with the voicing they like best and that best suits the tune. Voices come in a variety of different ranges and if the instruments are backing up a singer than they need to be in the key which the singer sings best in.