Opening .h and .cpp files from folders other than the sketch folder
I don't recommend this

A) Because the arduino editor is pretty bad for serious coding

B) Because you will need to have a header guard on all library code that doesn't have one already (even in .cpp's)

But, if you really have to, the only way I can think of is to make a symbolic link to the library file in the sketch directory. It works, but the file gets compiled twice, so you need those header guards on everything.