...will I still be able to - or how difficult is it going to be - pick up the pure mechanical aspect of my playing after 6 years of predominant inactivity?
Easier than if you have not done anything at all.
Due to pursuing school, the best I can get around to doing in terms of practice is 2-3 hrs every Friday/Saturday.
I think here you are not being honest with yourself, if playing piano was something you needed to maintain your very life you would certainly be able to practice other days too. In general 2-3 hours in 2 days is not as effective as doing a little every day. Development works best with consistent application of efforts rather than cramming.
When I do get to resume long hours of daily practice - which is not until 6 to 7 yrs later by which time I'll be 28 - how much of it is going to be permanently gone however much you fight to compensate?
First of all what makes you think you will have more time when you are older? What about family, work and other life interruptions?? Life always gets busy, if you feel too busy now you will always feel that way, it doesn't really change. People always say life situation makes it impossible to practice etc but it really isn't the case for the vast vast majority of people who say this. If studying piano is important enough you can make time for it every day.
You all should be no stranger to the common sense that age 20 or thereabouts is where the window closes on substantial technical abilities growth.
I don't see this happening when I look at the results from teaching students over the years. Sure most obvious and rapid growth is early on in your learning and you notice large leaps and bounds in progress which flattens off periodically as you reach closer and closer to your natural potential in various places, but you can experience this when you are far beyond 20 years old. There is certainly that "natural development potential" people have that is different and varies per individual, their natural ability to learn and play (talent), but hard work and efficient/continuous practice method development will propel you further past that (discipline). Someone might be considered "talented" when they play something impressive but if the observers knew the exact amount of work that goes behind it all they would realize it is more a display of discipline.
...I've heard said and been haunted by about how one's body starts plummeting the minute you're past your early 20s ...
Playing the piano for a lifetime is about getting to know your own hands and how to use them with as least effort as possible to produce your desired sound. If you require the body of a teenager or 20 year old to play piano then there is something wrong. You do not need to be an example of the perfect physical specimen to play the piano even at the highest level. I've seen plenty unhealthy pianists who play the piano like a beast.
...my physical mechanism is far better at seventy-two than it ever was in my twenties.... I find the above statement unduly pessimistic, far from common sense and quite wrong as it applies to me.
I'm with you on this one ted. Even though I was more physically energetic back then today I can do a lot more with less effort. The fear in the the OP to me makes me think about dancers where it is something often said even by the professional dancers that they feel they are on the decline and struggle to physically keep up with their abilties (though I'm sure there are those who break this mould). Piano is different though.