Firstly, do any and all processing in WAV files. If your source material is on CD, then rip to 44.1kHz/16bit WAV and process at 24-bit in your software. Do not transcode via mp3.
Tracks like 104b should be discarded. There is nothing you can do with files like this. The studio has not done their job properly. They have recorded the transfer with input ...
If the noise is induced in the pickups - eg from your monitor screen or fluorescent lighting, then no amount of electrical isolation will cure it.
The only cure is switching off the offending sources, or moving away from them.
A long-shot 3rd option is to get an old Line6 Variax, which doesn't use magnetic pickups at all & is therefore induced-voltage-...
You don't have a ground loop as there is no connection between the guitar and the computer chassis (other than via the interface).
You can try using a DI box - this might improve your situation but without having access to the setup it is hard to diagnose correctly.
Understand that sound-proofing is a frequency-dependent exercise. You can't simply quote an arbitrary decibel number without understanding what properties different materials have at different frequencies.
Frequency absorption is usually measured at 125Hz, 250Hz, 500Hz, 1000Hz, 2000Hz and 4000Hz.
Different materials will have different absorption ...
Soundproofing consists of several elements:
absorption: this stops sound from bouncing around inside the space you're working on. Acoustic foam is an absorption material.
insulation (soundproofing): this blocks sound from entering the space.
Styrofoam is not a good material for soundproofing, because it doesn't work for either of these.
Dynamat is an ...
Yes. Do not do this. You might get away with proximity if the signals were balanced, but with unbalanced audio, this is a bad idea. you are likely to get hum.
If running long distances, you might want to consider using balancing adapters to reduce rf and hum induction.
You should investigate the overlap-add technique of merging your audio buffers. It sounds to me that the buffers are windowed but are possibly just played out in sequence, causing the 'vibration' you refer to. Using an overlap add technique you should be able to smooth this out. Also check out the code for 'paulstretch' as this uses a similar synthesis ...