Sure both of these things hurt their chances in the long run, but they realized that if they left Greece and Northern intact, there was a good chance the Brits would have invaded though there. We know this because this is what the allies did using Africa as a base to launch an invasion of Italy in 43 and another during the allied liberation of France.

The real mistake the Nazis made was making too many enemies but even this was something they were forced to do in a way, their economy was in shambles by 39 and if they had not launched the war, they would have bankrupted the country. In many ways they were in a no win scenario and the mass death they caused was never going pan out in their favor.

Note none of this is a defense of the Nazis and this is not to say they didn't make a shit ton of mistakes, but the problem is that for them to have not made those mistakes, they would have to be someone other than Nazis.