too much stuff to address point by point, but in general, I think you are painting with too broad a brush. the audio field is truly filled with the spectrum from non-science to real science. and when you buy commercial stuff, you often have little idea where in that spectrum your gear's design and implementation fit.
otoh, with DIY gear you can verify its design and performance (like the whole open source thing). and so, I'll leave it there. if people want to do their own builds and then compare the sound of this kind of system vs their existing one, they're more than welcome. I do believe that if given a fair chance, the sonic clarity of the relay attenuator vs the typical $1 dual gang volume pot will justify the cost to the DIYer. what does a typical preamp (decent one) cost these days? then how much would it cost to build a DIY relay attenuator? the relay thing is in the $100 range, give or take what you want to do to it (PSU wise and buffer/gain stage wise). you can build it quite cheap (passive, just relays and resistors in the audio path) and nothing commercial can even come close in price or performance.
and finally, you get control over your UI when you go the DIY route. this is a non-audio side of the project but you'll never be able to modify or customize a sony UI, for example. and often, the commercial gear is very crass and raw in how its UI works.
I did used to think genuine analog pots were fine for audio use. after I did tests on typical pots and saw the tracking errors and crosstalk as the frequency went up - I changed my 'world view' on attenuators in the audio path. if you opened your mind and gave a new system a fair chance, maybe you'd see things in a new way as well.
the only downside, really, is that the relay attenuators are clicky as you turn the volume knob or run up/down the volume range. it runs a binary sequence and so each time you cross a binary boundary (31db to 32db, etc) you get a massive click via a bunch of relays being pulsed to 'roll over' the count. a few binary boundaries are noisy (lol) but not noisy in the speakers (I mute the relays during volume-change events). the relay chatter, physically, is noisy though. that's the only down side I can think of. all the rest are positives compared to conventional volume controls.