I'll start by saying that I've played with 4 different groups over 20+ years, and I don't remember ever feeling like a 15-minute day was a problem except back in my younger days, when we were just dungeon crawling for the loot and there was nothing else to the adventure.
For our part, I discovered the 15 minute day on the very first play of Keep on the Borderlands, when the magic-user spent his spell (singular) and rapidly grew tired of missing all the time with his dagger, before he was murdered by a kobold.
I guarantee that running adventure B would not feel like a 15-minute day. Plus, if adventure B led to adventure A, where they might have to rest once before defeating the BBEG, the players would probably not be disgruntled about a 15-minute day, because of the overall flow of the campaign.
I see your point, but I'd argue that this is no alleviation of the "15 minute day", because the scenario you suggest doesn't address some issues that make the FMD a problem for some groups.
First, part of the issue is that some classes feel the FMD effect much stronger than others. The sorceror feels it less than the wizard, for example. The cleric usually less than the sorceror. The effects of the FMD scale with time, as well. At first level, the wizard has nearly as good a chance in combat as most of the rest of the party. By fourth level, that's no longer remotely true.
Second, variability is nice, but it needs to be said that encounters under 3e can vary wildly in difficulty based on the party make-up. The CR system is a good
approximation of difficulty, but it is only that...approximate. Most undead encounters can be considered at least one CR higher if you have no cleric in your party, IMHO. A flying creature can be nigh unstoppable if the party has no ranged attackers or casters with the appropriate kind of spell.
Third, part of the function of the FMD is the lack of participation factor. Yes, the party
can go on, but may be disinclined to because it's harder and less fun to do so. If the cleric is out of healing and turns and about to enter a lair of the undead, they may wish to wait until they have those resources back. If the archer runs out of arrows, the wizard of spells or the cleric of heals, the party may have little desire to push on if the going looks tough. Yes, you can create a valid reason that they might have to push on, but you can't do that every time...it gets old.
To wit:
Fourth, you've kind of loaded the deck there. A has six rooms with EL equal encounters, but you make the assumption they're all combat encounters, while varying not just the EL but the content as well for B. The two aren't mutually exclusive (though B's fun factor is dependent on having a Rogue, someone with a good spot/search and most likely a cleric on hand). In fact, adventure B is very likely to be a FMD unless the DM makes it time-dependent, because it's quite likely that someone is going to get some negative levels from the wights. If there's no restoration magic handy, that guy could possibly be rendered useless. Lose a core party member to that and the rest of the battle is suicide, hence instant FMD.
I'm not of the opinion that the FMD is a show-stopper or unique to any version (in fact, 1e was far, far more guilty of it than 3e possibly could be). But I have encountered it plenty of times and it can suck the fun right out of the room.
Encounters are a tightrope, of course. Too easy and they waste valuable player time. Too hard and they frustrate and cause players to disengage. The "fifteen minute day" problem is one of the players being unwilling or unable to continue adventuring due to a lack of resources. This can come in a variety of ways, but the ultimate result is that the players, barring DM railroading, may choose to retire for the day rather than fly afield at weakened capacity.
How much of an issue that is for a group is entirely dependent on the individual gaming group and their playstyle. For some it might be a non-issue, while for others it could be a gamestopper.