The Ramban and "al pi derech hapshat"

I noticed this week that there is an interesting machlokes how many aronos were made in the Midbar. (which hopefully will be the topic of another post). The Ramban brings a pshat based on Chazal that there were 2,1 Moshe made temporarily and one that Betzalel made. Once Betzalel made his , the aron Moshe made was hidden.

The Ramban then says "al pi derech hapshat..." there was only one aron -the one Betzalel made.

How is one to understand the Ramban when he says "al pi derech hapshat". Is he arguing that the metzius was not like Chazal?

Coincidentally (although it is probably more of a case of hashgacha than coincidence) for an unrelated reason I decided to take a look in the Sifsei Chaim on Emunah and Hashgacha this week and lo and behold he discusses this issue. He brings another case where the Ramban argues on Chazal al pi derech hapshat-in Vayigash when Ya'akov comes down to Mitzrayim. Chazal say that the famine stopped when Ya'akov came down and the last five years started after Ya'akov died. The Ramban says al pi derech hapshat it doesn't make sense but rather even after Ya'akov came down the famine went on for 5 years.

The Sifsei Chaim asks, hayitachen that the Ramban would argue on Chazal ? He goes onto explain that even when the pashut pshat in the posuk contradicts the halacha or the metzius, it is still important to know the pashut pshat so we can learn a lesson from it. For example, by the eved nirtza-it says bring the eved to the mezuza or door. Halacha l'ma'aseh we bring him to the door not the mezuzah. If that is so why does the pashut pshat tell us to bring him to the mezuzah? The answer is to remind us of the mezuzah by yetzias mitzrayim that had dam pesach and that we should be an eved l'hashem and not to people.

In a nutshell the Sifsei Chaim understands that even the Ramban would be modeh that the metzius is like the drashas Chazal i.e. the famine stopped right away when Ya'akov came down or in our parsha that there were 2 aronos. The only reason the Ramban writes al pi derech hapshat is to help us learn a lesson from the pshat. (although what exactly the lesson is in these 2 cases isn't clear)

I told this over to someone and he said it makes no sense - how can you say the drasha is more emes than the pshat. I figure that the academics, intellectuals and rationalists would totally reject this approach especially the part where he says that the Ramban would never argue on a drashas Chazal. I am curious what people have to say about this.