You're right - I should have said 'done nothing until that point'.

That leads to the question 'how could he have followed them?' but that's not a terribly difficult question to entertain, as there's several junctures in the story where he could have safely transitioned into an 'observe from a distance' mode.

Certainly once landing in Egypt that's the case, but even the early Nepal detour becomes optional if you assume the Nazis would have continued to excavate everywhere until they found the Well of the Souls (or lost the upcoming war before doing so).

From Egypt-arrival onwards, if he'd kept an eye on things, then followed the bad guys into the desert, kept his eyes shut (he's the only one to know that was important) he could have transported the ark away without interference once that all went down.

So now goint meta: is this actually a fault? I mean this flaw is only in hindsight, from the perspective of the character everything was done right (at the moment to moment decisions without the gift of clearvoyance). Maybe the criticism shd be more that opening the ark is actually a deus ex machina (literally and figuratively), since nothing else could have saved indy in that situation).

Yup, and as Al-Khwarizmi says in a sibling comment, we are entertained through the journey.

Whether it diminishes your entertainment, once you get to the end of the story, to realise almost everything was pointless - in the fictional universe you just partook - well that's a question for you.

For me, it does not. But I also really enjoyed Joseph Heller's second novel.

A well-made film can also get away with one or two plot points that it doesn't bear to think too deeply about so long as the film as a whole is good.

The letters of transit signed by General De Gaulle[1] in Casablanca is a case in point. As someone once wrote, that would have been the equivalent of US forces being forced to accept letters of transit signed by Osama bin Laden.

[1]Maybe Peter Lorre actually says another name which would make more sense but De Gaulle is the general assumption.