Correct settings to record computer audio with Garageband, internal mic and ishowYouAudio
I believe the problem you are experiencing is caused by the computer CPU receiving to many command redirects. This delays the audio from being cached (stored) in the buffer at the proper time to be outputted. The lack of momentary audio data (samples) results in the static or clicking sounds.
Proof test 1: Does this problem occur when the laptop is cold and just turned on? If no, then this is the problem.
Is short you are asking the computer to do too much and as time passes the accumulative effect plus with the computer running warmer/hotter it will start to experience audio problems.
You will note that the problems first starts with your onboard microphone. Yet, the guest input remains normal with no problems.
Resolution 1: Poor-Maybe the computer is throttling back on the CPU because of excess heat. Do what you can to reduce the computers operating temperature.
Resolution 2: Increase the cache buffer size in iShowU. I am unfamiliar with this and a quick review did not lead to this being a possible alternative or not. This will create a delay in the signal. Find a happy medium if this option is available in your iShowU audio preferences.
Resolution 3: Flushing the RAM sometimes works. I use the free version of this "Memory Clean 2 (Direct Version)" and flush the RAM when static starts.
Resolution 4: Try using a different (light weight) browser.
Resolution 5: Amazon search this: Focusrite Scarlett Solo (3rd Gen) USB Audio Interface with Pro Tools | First. Then you need a XLR (balanced) microphone (no USB). This will be a $200 setup. You can use Pro Tools |First if you want as an alternate recording device. 

I experienced the same problems over a decade back. It doesn't matter how powerful your CPU is or how much RAM you have. This problem is not cause by the lack of either, it is a data flow problem. An external audio interface plus a XLR (balanced) microphone is a solid start to removing audio problems.

I noticed your voice needs a compressor on it to enhance the clarity of your speech. If you use the Pro Tools | First (free) included I believe it comes with a compressor as well as other virtual audio hardware (EQ &amp; etc)you will adopt for your productions.