That may be true to a large extent, the engineers surely know... a lot more than they would prepared to tell publicly, in any case they wouldn't come out with stuff to slag their current drivers, even if they had facts to support it. I know RB has come in support of MS and blamed the tyres, yet I'm not convinced, he's basically in the same position as Norbert Haug, long-time buddy of MS and instrumental in bringing him back.
I'm not sure if I understand what you mean, but there are a lot of people working in the engineering department addressing the problems according to the telemetry data - and if the team bosses official party line would consistently somehow be against the data, I don't know how long they would be able to keep the sham up. There are investors to report to, constant changes in personnel and also lot of internal competition inside the team, so the truth would eventually creep out. By constantly lying about such a thing the bosses would make a lot of people unhappy and it would in the end in a normal situation cause a lot of harm both inside and outside the team.
And even if we buy into this evil tyres theory, it's not really a valid excuse, not for a supposed star driver. They are supposed to adapt and get at least as much out of the equipment as the guy in the other car, even if they'd prefer equipment with different characteristics. In fact that's what we witnessed with Alonso in 2007, in all likelihood he would have prefered to stay on Michelins and be far more at ease, yet did he complain? Not really, all I remember is him saying that adapting to the Bridgestones is difficult and the main area to work on for him. That's what I'd expect, not "Oh, it's the tyres, they're shit, nothing to do about it than to wait for more suitable ones next season, maybe, hopefully."
But isn't what Schumacher has said just the same that you remember Alonso has said? Surely MS has said "he's working on it", only with the exception that the tire supplier wasn't about to change in 2008, so Alonso couldn't say "hopefully the next year's new tires work for me better", which I'm sure Alonso too would have then said at some point?
Furthermore I'd be cautious with the expactations regards "star" drivers - there really isn't any way to determine what a driver "should" be able to do in a certain situation, because the cars, tires, teams and yardsticks constantly change and every season becomes a different situation. Except a blanket statement that the "stars" should prevail against their team mate regardless of the situation. This in turn I'm certain is false to begin with, as any star driver past or present would get beaten by a teammate considered "lesser" in a legitimately "equal" situation, if the cards that we can't see stacked up against them just right. That's the unfortunate nature of the current motorsport formulas.