As you were thorough, I will try to be as well. A couple of things:
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See if there isn’t a “Master Volume” inline control on your headset.
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I have had mixed results using a 1/8" (3.5mm) to 1/4" headphone adapter. My best results were with a 3.5 mm TRS(female) to 1/4" TRS(male) Stereo Headphone Adaptor Cable. You might have a similar issue with the microphone.
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Your guitar looks as though it has a Hi-Z output. It is good that you have selected an interface with a Hi-Z input, so it looks as though you have done your homework.
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You may need an expensive guitar cable (about $30). The $10 cables will give you annoying hum or distortion (no extra charge). However, I suspect you already know about this and it is probably not related to your specific issues.
You have described two issues:
I assume you have adusted the volume and tone controls on your guitar, the “Master Volume” inline control on the headset, and fiddled with headphone adapter. Do you have another set of headphones/earplugs that you could try?
You pointed out that you saw the other Audacity post, “VERY loud white noise” coming from the Subzero A12:
Considering we have seen two posts on the A12, both complaining of white noise, perhaps there is a systemic problem there. Have you contacted their customer support?
I see you have downloaded the driver for the A12; I was going to suggest that. As you do your research, note also that the A12 is also sold as part of package, the Subzero base-2. Good Luck!