Firstly, there's no reason to assume that 5e won't include its equivalent of "page 42". WotC are well aware of how well that was received in 4e; they would be mad not to include something similar.
Secondly, the trade-off for the 4e rules being "very clear" as you say was that the core rules ran to nearly 1,000 pages, and the supplemental rules ran to several thousand pages (and the game was structured to strongly encourage the use of those supplements). Personally, I don't find that a worthwhile trade - I found keeping track of that mass of rules (not to mention the constant changes) far more taxing than the need to make rulings.
That was not my experience with 4e at all. But I really don't want to rehash the 3e/4e Edition Wars, so I'll stop there.
Oh well, in that case, you're absolutely right! I had forgotten there were five whole numbers to work with!
Seriously, the difference between three and five levels is trivial.
Again, you assume that these guidelines won't exist. Funnily enough, until 4e was released there was no reason to think "page 42" existed in that edition; there's actually more reason to think it will exist in 5e (since WotC know it's a good idea).
So...?
Not only do I not think that's necessarily a bad thing, but neither do I think it's a new thing. Different DMs have always used different subsets of the supplements, they've always used different house rules, and they've always applied the rules that they do use unevenly. No two tables have ever been exactly alike, even in tournament play.
If it bothers you that much, write down your rulings.
Alternately, you could note that the exact same situation doesn't actually come up twice in the campaign. The PCs gain experience as they go, and they never again meet quite the same door a second time - either its a subtly different door, or if it is the same door then it has been changed by the previous interaction with the PCs.
In other words, while the DM should strive to be consistent in his approach to the game, he doesn't need to be perfectly consistent.
Besides, the players are likely not to remember either. In my experience, they find it a challenge to remember what happened in the last session even in broad strokes!
This is a misconception. Advantage and disadvantage aren't bonuses at all; they're rerolls. They will neither let you hit a DC you previously couldn't, nor automatically take one that was previously possible out of reach.
Actually, unless I've misread something, the DM shouldn't be doing those probability calculations at all. The DC of the challenge should be set, possibly in relation to PC capabilities, but independent of the possibility of advantage. Then, when the situation occurs, advantage may be applied based on the circumstances (and good play), but this should not be a factor when setting the DC.
That way, if the players are able to play well, and so gain advantage, then this serves as a very significant benefit. Which is as it should be!
This is no less true of the +2 bonus. In fact, it may be more true, given that that does allow the PC to hit the previously unhittable. And those +2s very quickly add up. Because advantage doesn't stack, that problem is avoided.
That's no less true if throwing sand reliably gives a +2 bonus. And yet, somehow we've made it through 40 years and multiple editions without every PC carrying a bag of sand, and one of flour to beat invisibility, and a mirror to beat the medusa, and string for those mazes, and...
But, just in case this is a real problem that you're facing, here's a solution: Point out to your players that their characters exist in a fantasy universe, overseen by closely-involved and fickle deities. Deities who love to be entertained, and who enjoy both boldness and cleverness. So, the first time they saw the "throwing sand" trick, it amused them enough to give Advantage. But if you do it again, it's not so exciting - they start to get bored. And when the gods yawn, that's when Disadvantage gets applied, even for the exact same trick.
(Incidentally, doing this also has the side effect that the players will now start seeking out ever more inventive ways to entertain the 'gods' with their antics. Which has the happy side effect of making your game more entertaining.)
There. Job done.
How is it a good thing for players (mature or otherwise) coming back to a game they don't like?
There's nothing in 4e stopping the players from entering the dungeon, using all their Dailies in the first encounter, retreating, and resting. Any solution that the DM employs to prevent it will also apply to pre-4e editions, and to 5e also.
Plus, WotC have already noted that with 5e they are taking steps to deal with the 15-minute AD - by setting an XP budget for the day, rather than the encounter. That way, if the PCs use the 15m/AD, the DM just responds by rearranging the encounters - instead of 16 challenging encounters, they get 4 lethal encounters... either way, across 4 days of play.
As for scry/buff/teleport, you're again assuming that WotC won't have put in counter-measures. Besides, the validity of that tactic in pre-4e was always exaggerated - even in the core, there were plenty of counter-measures.
That may be what you think you're doing, but you're really not. What you've done is taken genuine concerns, and quite possibly valid concerns, and wrapped them up in a huge amount of hyperbole and edition partisanship, to the extent that your point has become thoroughly obfuscated.
The problem is that you may well be right, but the manner in which you have expressed your concerns means they're much more likely to be dismissed than dealt with.
