@Santiago_KinderBueno
You make many interesting points about our full backs in van Gaal's current system. What really caught my eye is how they're supposed to be more central rather than wide in their attacking approach. It reminds me of the 1990's Ajax team under van Gaal who had centrally-oriented full backs in Reiziger and Frank de Boer with Rijkaard stepping up to support the attack and Davids + Seedorf drifting out to the channels to stretch teams and support Finidi and Overmars.
The thing about that Ajax setup, though is that it had a libero stepping up, two box-to-box midfielders, and one #10 in the midfield. This is a setup that van Gaal hasn't replicated since then. In the 21st century, van Gaal changed his system with inside wingers and wing backs becoming more prominent. With Bayern, we've seen him play a 4-2-3-1, regularly playing an attacking full back like Lahm on the right side and bringing up Contento in order to move back Badstuber to central defence. When van Gaal was playing the 4-2-3-1 with the Netherlands, he had two attacking full backs. With us, he played a back 3 and encouraged the wing backs to really bomb forward and provide the width, and he continued that on with our backwards-pointing 4-3-3 in which Blind and Valencia both overlapped the wide players.
This is not to say that he prefers attacking wing backs to play the full back roles. He has still set up his full backs such that his system is not compromised, and it's the most balanced approach that he's set up for his full backs (which further highlight's van Gaal's tactical nous and creativity). As you see in most teams nowadays, once they have the ball, the full backs push up to become wingers quite quickly. To support them, the central defenders push out wide, and the defensive midfielder drops back to make a back 3. Ahead of him are attacking midfielders/inverted wingers and one attacking midfielder and one box-to-box midfielder to supply the ball to, who will move the ball forward and supply players in the final third.
What van Gaal does, however, is completely different. He doesn't have his full backs push up with a defensive midfielder dropping back. He has his defensive midfielder remain ahead of his central defence whilst the full backs stay back. As you said, we aim to build up centrally through the central defenders and defensive midfielder, spreading out the team as we advance forward and utilising the wingers as a result. If we have a destroyer and box-to-box midfielder in a 4-2-3-1 (van Bommel-Schweini, de Jong-Strootman, or Schneidi-Herrera?), he will have the box-to-box midfielder carry the ball forward with the destroyer providing support in the build-up. If we go with a backwards-pointing 4-3-3 or the Ajax 4-3-3, he'll have the defensive midfielder/libero (Carrick or Rijkaard) dictate the build-up.
Full backs in van Gaal's setup are very important in the build up of attacks. They are key in helping the team maintain possession and move the ball forward through the middle. van Gaal doesn't want them to be adventurous wing backs and break the lines regularly like Alves, Zabaleta, Coleman, Alba, etc. He wants them to provide support to the central defenders and midfielders during the build-up. The full backs are key in overloading the wide areas and creating spaces through the middle. Such players are expected to be very good in retaining possession and playing midfielders into space in the middle. We saw this happen many times with McNair, Blind, and Valencia whilst Shaw is still adapting to this. Having said this, in adapting his system, van Gaal also wants his full backs to provide the width higher up the pitch when the wingers go inside. We saw this many times with Blind going forward to support Young, Rafael on the right when we played with the diamond, Valencia on the right when Mata drifted inside, and Shaw bombing forward from the left. It's not the first time he did this either; he did this before with Lahm and Contento at Bayern Munich, and Janmaat/van Rhijn and Willems for the Netherlands.
As a result, don't expect him to remain with his old-school approach; I'm sure that he won't mind using an attacking full back as long as he shows discipline in his approach and doesn't bomb forward/break the lines too soon.