I was going to address your post point-by-point, but I think it's probably better to start over. It sounds like you've done quite a bit of workshopping here. I disagree with a number of the conclusion's you've reached. If you really do want a juicy steak beyond well-done, I don't think you're going in the right direction to get the results you're looking for.
In a steak— which for the sake of this answer I define as a quick-cooking cut of beef, rather than the more accurate definition which specifies which way it's cut— the thing that makes it 'juicy' is the moisture that's trapped in the muscle fibers. When that muscle fiber tenses up, either through heat or acid, it squeezes out the moisture. Asking how you can keep a steak juicy beyond well-done is like asking how to keep a bag of sponges wet after you've rung them all out. The most likely explanation for your vinegar onion mixture keeping the steak from shrinking is that it already shrank from the acid in the marinade before you put it on the grill.
Enzymes will only succeed in killing the structural integrity of the muscle fiber. By the time they're decomposed enough to not squeeze out the moisture when they're cooking, you're going to have meat gruel rather than a steak. If you're interested in eating mushy steak, you could probably succeed in using enough of an enzyme like papain (which is usually what's in the 'meat tenderizer' jar in the spice section of your grocery store) to do that for you.
Taking a standard steak cut, marinating it, and grilling it ded is going to produce a dry, powdery, and/or mushy piece of meat every time.
Now, consider a piece of braised beef. It's FAR beyond well-done, (over 190F/88C) but it's pleasant to eat. This is because good braising cuts— think brisket, short ribs, chuck, or anything that would go in a beef stew—have a whole lot of the connective tissue collagen, and when cooked gently for quite some time, that collagen dissolves into gelatin. The muscle fibers themselves are quite dry and stringy, but each 'bite' is juicy thanks to all of that gelatin packed into the meat in between the muscle fibers. If you were to take some shredded, braised pork and rinse it in some water, it would immediately cease to be a luxurious rich shredded meat, and become a pile of dry stringy garbage.
The only way I can think of having beef beyond well-done on the grill which is not extraordinarily unpleasant to eat is to use a cut like chuck, lightly braise it first (you don't want it falling-apart tender,) let it cool in the braising liquid, and then finish it on the grill. It will keep in your refrigerator quite nicely after it's braised, and you can reheat it on the grill; no need to heat it beyond 135 or so when reheating it, so it won't dry out. If you reduce the cooking liquid, it will make an incredible glaze that you can brush onto the meat as it grills. If you're looking to get more of that raw-steak-on-grill flavor, sear the meat on very high heat on the grill before you braise it. The flavor will be infused throughout the end product.