Why would the line be straight? It appears that the matter wave of an electron wraps around in the orbitals of atoms. The matter wave would be in the direction the particle travels.

I indeed thought too fast and was focusing on the 1D case. With a period being infinite, we can see the 1D wave as a straight line. Which is what I had actually in mind.

While the wavelength of a matter wave is independent of electric charge, it's path should be influenced by electric charge

The wave vector follows the momentum, or vice versa. So yes, somehow.

n the double slit experiment, the size of a matter wave must be large enough to have some part of it go through both slits when an electron goes through one of the slits.

There is actually no way to answer that question. The electron passes through both slits at the same time. You may want to check some of the older posts I wrote on this.

I think you are playing with many particles like that at LHD, yes?

Here you also need field theory and relativistic mechanics, but yes we do :)