My opinion is also based on some experience in this field - 20+ years playing and still one of the fittest in any team I play in :-)
I agree that we should have been back earlier to build base fitness but disagree that a friendly between the Europa games would not have benefitted the team.
I don't get the point about being more knackered if we played a friendly. IMO, for a professional athlete to be tired after 30 mins against a part-time team is a disgrace at any time never mind 2 games in a row. How could playing a friendly in between make that worse? The 'training' obviously didn't work did it?