[1] I assume that this variation in ADCs and DACs use must also vary to some degree between tracks due to differences in mixing for each track on many albums.
[2] Compression today really drives me spare, as while think I understand the commercial justifications for it: perceived louder is 'better', at least in the market, it so destroys the dynamics of the music.
[3] If all hi res was this less compressed, compared to CDs, it would be worth replacing all my 1000s of CD rips for the better dynamics alone!
1. Generally, though not always, the various ADC/DACs used on one track will be the same on all the tracks on the album but not the number of round trips through those ADC/DACs, which could vary wildly between the different tracks. All MQA attempts to "fix" is one pass through the recording ADC and the pass through the consumer's DAC. Note that this mainly affects recording older than about 5-10 years. Today the typical workflow is ITB (in the box), meaning one pass through the recording ADC and then all processing is carried out in the DAW (digital audio workstation) without any further D to A or A to D conversions for analogue processing. This isn't always the case though.
2. This isn't quite as simple an issue as it appears. To try and simplify, there's two parts to it:
A. High levels of compression (destroyed dynamics) is actually "better" in many situations. Many consumers listen to music while doing other tasks in extremely noisy environments, such as when working, travelling, exercising, doing chores, etc. Wide dynamic range recordings are completely useless in these situations because the quieter parts will be beneath the noise floor of the listening environment and inaudible. For example, I don't listen to classical music when driving because I could be sitting there for minutes not hearing anything at all during the quiet passages in the music and I don't want to be turning up and down the volume all the time as the music gets quieter and louder. So I just listen to popular music genres which are heavily compressed when driving and at least I can hear it all. Listening critically though, for example when I'm at home doing nothing else but listening, on a decent system in a moderately quiet listening environment, then destroyed dynamics is very annoying or at least far less preferable than wider dynamics. So, there is a good justification for two masters, a highly compressed one and a far less compressed one or if only one master, for it to be highly compressed because music is consumed by far more people in noisy environments than there are audiophiles.
B. Economics! The problem with the CD container format (16/44) is that it's been around for a long time. That's a very serious problem economically because once a consumer has bought a decent 16/44 DAC/DAP/system there's no real need to buy another one until it wears out, and this isn't just an equipment problem but also a problem for content owners (record labels) and distributors. That's why "high-res" consumer distribution formats were invented, you had to buy new equipment to play "hi-res" and obviously you have to buy the content/music again or buy new content in a more expensive "high-res" format. The reason I put "high-res" in quotes is because in practise it's NOT higher resolution, "high-res" results in EXACTLY the same resolution as 16/44 within the audible range. This inconvenient fact can be overcome for some consumers with marketing, however, that marketing is far easier and the number of potential consumers is far greater if there is a real audible difference. This brings us back to the point above and the justification for two masters, release the highly compressed one in 16/44 format and the less compressed one in high-res, now we have a real, measurable and audible difference between high-res and 16/44 and in critical listening situations the high-res version should always sound superior! Of course, because there is no resolution difference, they could release both versions in 16/44 and the less compressed version would be audibly indistinguishable from a "high-res" version but economically that doesn't serve any section of the industry's best interests.
3. As just explained, you buying your entire music collection again is one of the main reasons hi-res exists. Record labels have literally billions of dollars worth of back catalogue content and they want/need to make money from it. Sometimes it's not technically possible to create a far less compressed version for "high-res" release, in which case you either have to add even more compression and/or create a new (or existing master) which is different in some other respect. Therefore, you cannot guarantee that repurchasing your collection in "high-res" will always get you a less compressed version.
We may be in danger of getting kicked out! Although arguably obliquely related, what we're discussing now is pretty far off topic.
G