MPP trackers are essentially switch mode voltage converters, with intelligent software that juggles the load on the Solar panel
to obtain the maximum output power.
There are many common algorithms used for the load juggling, but basic idea is to measure the output power from the Solar panel
and then change the load in some defined manner, and then see what happens to the output power.
Its not that hard to do, but you do need a good understanding of how to design switch mode power supplies to keep their efficiency
high.
For very small Solar panels is not worth doing,as the output power increase is at best around 30%, and unless you can source all the parts
for the tracker for next to nothing,its usually cheaper to just buy a 2nd solar panel.

What sort of Transmitter and  Receiver  are you using.
When you say the receiver is not responding, what do you mean.
How are you looking at the receivers output.
Actually measuring the Transmitters frequency needs either a frequency counter or spectrum analysis tool
which most people dont have .

As above, and the other problem with trying to use a hardware usart with simple radio modules
is that in the absence of any transmission , the receiver outputs continuous random data transitions
which are not at any constant baud rate , and this will cause the usart at the receiving end to continuously
generate overrun and framing errors, which must be cleared before it will work properly.
To do what you want using radio needs intelligent Transmitters / receivers which will simulate correctly a hard wired
connection.
Look at using Xbee modules, some of which can do this.

It sounds like the OP has some form of Efergy Energy Monitors.
This article may be of some help.
http://electrohome.pbworks.com/w/page/34379858/Efergy%20Elite%20Wireless%20Meter%20Hack

The only practical way of extracting data out of these types of display devices, is to extract the data
from the wireless receiver chip and then decode it .
Its generally not possible to get at the decoded data once its inside the Micro without having some detailed
information on how the Micro works.

What exactly are you trying to achieve with this measurement?
A Solar Cell is a current source , and will produce 0.5 V per cell even with only a low light level.
So a Solar panel , even in a relatively low light environment will produce close to its full voltage.
The voltage wont change much , even though the light level may change a lot.
To measure the change in light level, you need to put some kind of load on the Solar Panel and then
measure the current.

Have a read of this , will help you get started.
http://www.open.com.au/mikem/arduino/VirtualWire.pdf

What type of Transmitter is being used?
If its a simple ASK Transmitter , then leaving the data pin in a high impedance state can cause the
transmitter to intermitantly transmit , as the voltage on the pin will float around.
Much better to leave it with a 0 on the data pin.

The Inductors used in boost converters need to have a maximum current rating without saturating
of at least the maximum output current of the circuit X the ratio of the output volts/input volts.
An inductor that small will be saturating, and ceasing to be an inductor.
Needs to be a lot bigger.

Its unlikley to be a problem with the relay, if resetting the Arduino releases it.
How is the Arduino powered?
Is it from the same 12V that runs the motor?

If you really want to measure energy accurately, you are far better off using a device specifically
for this purpose like a ADE5569.
http://www.analog.com/en/analog-to-digital-converters/energy-measurement/ade5569/products/product.html

It does all the hard work for you.

What sort of accuracy are you expecting with this energy meter?
Current only sensing energy meters can be highly inaccurate as they dont measure the voltage.
So with loads that have a power factor of less than 1 , the meter will read incorrectly.
Loads that have switch mode power supplies in them , like computers or plug packs will read even more
innacurately, due to the current waveform not being sinusoidal.

Depending on how cheap you mean, you can buy spray cans of circuit board lacquer
that you simply spray on the board after its finished.
Its non conductive and adhesive and protects the board.
Made by Servisol, and most electronic hobby stores sell something like it.

You cant measure voltage with a current clamp or a current sensor.
This sort of project is not for a beginner.
To accurately measue the energy consumption of an appliance you must measure the voltage and the current and the
phase angle between them , as not all loads are purely resistive.

At the low currents your chips need, a simple series resistor and a 3.3 V zener diode is all you need.

You will need some intelligent processing to measure the output of each force sensor and to drive
the 433 Mhz Transmitters.
Maybe not an Arduino for each sensor, but some kind of microcontroller to read the sensor and to transmit the data.
Those little 433 Mhz Transmitters are totally dumb, and cant be addressed in any way.
You need some intelligence at both ends.
Have a look at the Virtualwire library which is designed specifically for these Transmitter / Receivers.