On question (a) the field is the force per unit charge. Force is mass $\times$ acceleration. Acceleration is the rate of the rate of change of position over time. And since you can change position in more than one direction, this quantity has to tell us about direction. As such, the force and the field also have to be directional quantities and two different forces can be perpendicular.
The wavelength of a single photon refers to something that will take some setting up. Suppose you shine a laser beam at a filter that blocks some of the light. This will reduce the amount of light in the beam after the filter. As your put more filters in front of the beam you might think the amount of light in the beam will decrease gradually, but that's not what happens in reality. In the real world, if you have a sensitive enough detector there will come a point at which the detector sometimes goes off and sometimes shows doesn't. And for intensities above that level the detector will go off as if the energy comes in chunks.
Now, let's suppose you shine just one photon at a time through a pair of narrow slits a suitable distance apart. So if you put a detector in front of each slit at most one of them will go off at any given time. If you shine many photons through the two slits one at a time you will see a pattern of bars. Light bars will have lots of photons arriving, dark bars will see few photons. If you block one of the slits, you see a different pattern of bars. This means that by unblocking one of the slits you prevent photons from going to places that go from being a light bar to being a dark bar. So there is something going through both slits that changes the pattern of bars. You can perform experiments to test how the thing going through both slits behaves. It is deflected by lenses and mirrors as a photon would be. It is blocked by opaque objects as a photon would be. It is a photon in every respect except that you don't see it. A photon exists in multiple versions and you only ever see one of them directly. There is a wave like thing called the wave function that consists in part of the multiple versions of the photon whose intensity is related to the probability of seeing a photon at a given location. The fact that the probability can decrease as a result of allowing another source for the wave function is a sign that there is something more complicated going on than just the existence of many versions of the photon. The more complicated stuff is called by various names like phase and entanglement that I won't go into further here.
Now the different versions of a photon are affected by lenses and mirrors and other stuff. The effect of the lenses and mirrors is local: it affects the wave function in its vicinity. And in general changes in the wave function propagate by affecting the wave function first near the disturbance and only later does it change the parts of the wave function further away. That's how the wave function propagates. Changes in a particular region gradually spread out over time by affecting the wave function in nearby regions.
There is much more to say about this, for which you should read "The Fabric of Reality" by David Deutsch, especially chapters 2,9,11 and "The Beginning of Infinity" by Deutsch chapters 11 and 12.