Malachi256:
1. In fact, the famous Charlie Company in WW2 was reinforced several times to full or partial muster during their campaign through Europe. Healing through fortification could be counted as waiting for partial reinforcement to occur. Insta-Heal could be counted as reinforcing the unit to full muster at any and all costs. It's not quite the break from reality you might think it is, I'm afraid.
2. Somewhat. I'm not either for or against the promo for myself. It's useful enough every now and then, but my main "use" for it is so the AI doesn't keel over quite as quickly as it normally does.
It's sometimes desirable to swell your forces with a unit or two purely for taking damage (and then you disband them after the war). I typically use Military State forces for this, but in the absence I sometimes build a green unit or two for the purpose. It adds another wrinkle to the strategy. A more promoted unit may take less damage in the long run, but without Insta Heal, it doesn't soak mass damage quite as well as a green unit. They're literally cannon fodder, in the Industrial Era.
3. I don't generally agree with Firaxis' way of providing AI bonuses by way of adjusting the difficulty settings. I'll accept that up to Emperor, but above that it just skews play way too much - to the point where players are settling on luxury resource purely for the benefit of selling them to the AI (and building ND for similar reasons!) In fact, I generally play on King. If I find an AI weakness, I generally try to avoid playing to take advantage of it. I have a lot of personal house rules, as you can imagine.
4. It's more timing the attacks so that the unit dies on the same turn. That's not altogether that alien of a concept. In real terms, you could say that you're coordinating attacks so as to induce a rout rather than a timely retreat from which the enemy could regroup.
There are better things you could be doing with your army than feeding XP to the enemy, after all, Insta-Heal or no.