Electron interference
Traditional understanding of trajectory is of no use for electrons. Granted, on macroscale you may detect something close to movement and trajectory, (electron beams and electron cannons take their names from this) but it still far from what we have in classical mechanics. Ideally you should abandon any attempts to relate quantum and classical mechanics and view quantum mechanics as an abstract mathematical construct.
Incorrect question. If you have a system of two or more particles, you should consider the wavefunction of combined system, not separate wavefunctions for separate particles. Still, for electrons being fermions, there is a strict rule that wavefunction must be zero for positions in phase-space where two electrons of same spin are at same location and since electrons are of same charge, probability of having two electrons of different sping close to each other is usually lower than to have them at some distance.
Yes, only positive probabilities. Though, this does no apply to wavefunction itself, it may be negative at some point (actually, its value can be a complex number).