Well, if the PCs only use their "encounter" abilities, they can get back into the dungeon after a day's rest.
Yes. Take it this way - take normal play. Every time PCs would stop for a few minutes, they are now stopping for the day. Every time they'd stop for 6 to eight hours of rest, they are now stopping for a week.
Most adventure designs I have seen expect you to be using encounter powers in every encounter. That's kind of the point of having them recharge on that timescale. So, after every encounter or two, the party will have to stop and rest for 8 hours.
In addition, why would monsters necessarily move in faster than the rate that the PCs can clear them? A world with a lower population density of monsters might see much less movement than "standard" D&D assumes.
Assume a normal game, where the PCs could have come back the next day. if the PCs chose to walk away for a week, would the DM keep the situation static? Probably not.
Look over the Kobold Hall in the back of the DMG. For most of the encounters, you can posit that the PCs can take a few minutes break between encounters and have it play out as written. If they leave for an entire night, though, the occupants are going to know something is up, and the thing won't play out as written.
It is vaguely plausible that the PCs could leave for the night just before the final encounter, and come back fresh (and really, if they don't, they're probably hamburger). It is not at all plausible that they can disappear for a week.
Again, this may be a factor of how quickly a large organization can bring its strength to bear on the PCs. Perhaps the organization's more skilled members are widely scattered, or the PCs initially take on a small branch of the organization far from its center of power.
Remember, we are positing no change to adventure design. To first approximation, the "large organization" is the contents of whatever dungeon, thieves' guild, or wizard's tower the party was entering. All that force is available immediately. And teleporting away is not generally an option until 8th level.
Thus, for some reason if they party is to survive, the "large organizations" must not be particularly large, or must be diffuse, as you suggested. However, having a logical rational for every large organization the party's apt to deal with to be organized that way, when the heads of said organizations
know how slowly powers recover, will take some doing. Bit of a plausibility stretch, if you ask me.
The general thought is as follows - there's some basic timescale of communication among the enemy. "Adventures" need to be no larger than what the party can deal with in that timescale, or it will not play out as written. There is no point in statting out a huge dungeon with all it's populace in detailed locations, if all that population is going to reshuffle after the party has gone through three or four encounters.
This is true regardless of system - in Shadowrun, the typical incursion has a timescale of minutes to an hour, because that's the response time when the bad guys have radios and telephones and cars. And because if the party is in there for more than an hour, they are probably dead.