Spy
We can go-round some more on this when the official report is published. A problem with discussion in a substantially fact-free environment is that the same hypothetical oil keeps lubricating increasingly circular commentary.
On the surface of it, the misuse of crossfeed - for a long enough interval to sacrifice the critical fuel reserve - seems like a rookie oversight. Surely there is a very specific explanation for this - which we do not yet know as a matter of fact.
Other than for passing time in idle chatter, the interest in understanding the underlying failure of performance of the aggregate process is directed toward improving systems - whether mechanical or electronic or human - to lower the probability of a similar or analogous reoccurrence. Since the frequency of events like this is mercifully low, each one deserves a great deal of attention and introspection before the 'causes and effects' account is closed out.
Your explanation tends to focus on a presumed defect of skill or character or alertness on the part of the two flying crew members. While that may be correct, at some level, it is also true that all humans are prone to occasional excursions away from perfection in the conduct of their duties. That is the main reason we strive so hard to wrap multiple levels of systems redundancy around humans who are working in critical-path processes. Somewhere, somehow, one or more of those protective envelopes has failed dramatically here - with the drama heightened a bit by the cool and focussed way the crew handled and resolved the final mess they were in.
I find it particularly interesting to consider whether the 2-person ETOPS crew had adequate resources to identify and diagnose their problem in the time that was available. The facts seem to show they didn't. Your position, if I may presume to restate it, is that they "should have" been able to solve the problem with the means at hand. But one round in the chamber trumps ten on the shelf: the fact they did not resolve it as you suggest says prima facie they lacked some critical insight or datum at the moment of need.
Historically, pilots have had to work with a a chronic shortage of information about what is happening inside their aircraft, along their flight path, with weather, traffic, etc. Just quite recently the tables have turned so that vast amounts of information are available - in very current form. The connundrum has shifted in a span of decades from figuring out how to make up for missing data to figuring out what portion of a mountain of data to use and which to ignore. Perhaps this is a pivotal factor in the uninterrupted progress of Air Transat incident?
The extent to which the flying crew can also be maintenance technicians is controlled by the quality and fitness-to-task of the tools available and also by the mindset of the individuals. The perspectives of the decisive aircraft Commander versus the skill-rich and very time-sensitive Pilot versus the plodding but persistent diagnostic Technician are somewhat mutually incompatible in exactly the sort of circumstances that occurred. The pre-dawn hour and mid-ocean location provided a near worst-case scenario for access to ground-based backup resources, complicating matters further. In any what-if scenario one can cook up, the cycle probably could have been interrupted for a more favorable result if a highly-skilled and authoritative 3rd party or team could have gotten inside the process by radio and telemetry early in the situation.
What-ifs abound here. This incident was/is both symbolic and symptomatic of the transition from one era of aviation to another. It would be an opportunity wasted to write it off simply as a 1-dimensional bolix.