The original Schlieffen Plan released in 1905 called for what is known as a 'swinging door' strategy. The Germans would throw literally all they had through Holland and Belgium to rapidly get into North France in a surprise attack, conquer Paris, and goad the French into conquering parts of West Germany. As they were sufficiently over extended the Germans would swing in from behind with all they had and crush the French from behind giving them no retreat. This combined with a quick stealing of Paris and the French would be forced to surrender.

Alfred Helmuth (Careless errors!) von Moltke would radically alter this plans in the face of a changing landscale. The original Schlieffen Plan was idealistic -- it was a perfect plan for a perfect world and a perfect timetable. It did not account for human agency/mistakes and further, it allowed for many parts of West Prussia and Bavaria to be taken as a sort of sacrificial lamb -- something many people did not like. The issues were as follows:

• He would note that with violating Belgian neutrality, Britain would almost certainly join the war effort which meant a blockade. The Germans were not breaking the blockade. The Germans would be starved of foodstuffs and war materials if they were blockaded. I think you're getting the picture here. The alternative he presented was that the Netherlands would not be invaded, Holland left safe, and Belgium the only country with its neutrality broken. This was done so that German could have a neighboring trading partner with open ocean access to get foodstuffs in particular from.

• On top of this he would recognize that the Russians would not take almost 2 months to mobilize and could present a far more immediate threat. In combination with this he also realized that the idea of purposely and willingly giving up German territory was something impossible to ask for, particularly because those who were to be intentionally retreating and giving up Bavarian land would be the Bavarian Army primarily and their Crown Prince performing the operation.

The plan would be altered from a 'swinging door' into a squeeze strategy. The Germans would oppose French incursions directly and perform a steadfast defense. They would not allow an inch of German territory to be given up willingly and would have bolstered defenses to do this. What was once an 7:1 ratio of Germans in the "hammer" and Germans defending in the "anvil" would be reduced to 3:1. Once the war began the plan would also be changed in the flow of battle as well as the defense against French assaults were not only stunted but crushed; the Bavarians wanted to push into the heart of France in counter-offensives. Combine this with a Russia mobilizing faster than even Moltke expected and 'the hammer' was bled almost entirely dry of its 'swinging' force with a further combination of a brilliant withdrawal plan by the French.

Basically I don't say Schlieffen Plan as what Moltke enacted was not the Schlieffen Plan but something inspired on it. As there is no historiographical term yet to show this I just go with Schlieffen-Moltke Plan for what Moltke enacted and even that plan was 'tainted' in the process of the war. Kluck, Bulow, and Hausen had absolutely no communications (largely in part due to a shitty telegraph structure by the Germans with next to no telephones along with an extraordinary effort by the Belgians who routinely cut telegraph and telephone lines). This meant that even what plan they had had to be improvised upon numerous times as situations arose in front of each of them individually but they had no way to communicate with OHL (Supreme Army Command) or the other two of 'the hammer'. This would ultimately lead to Bulow ducking while Kluck dove, giving a massive gap, and the Parisian garrison/BEF nearly encircling the 1st and 2nd Armies.

Did Germany make major mistakes in getting through Belgium and whanot? What were these mistakes?

It's hard to determine 'mistakes', even retroactively. If anything I'd say the brutal oppression of the Belgian people was their greatest mistake as what little opposition to the war remained in '14 and '15 would be quashed by the propaganda goldmine of what is affectionately called the Rape of Belgium. 500,000 Brits would volunteer in 6 weeks and 2.5 million total would volunteer in 1914/15.

Can we say certainly the Rape of Belgium helped put popular support and volunteer numbers into overdrive? No but it was certainly non-negligible. It also, in Belgium, gathered massive support for guerrilla tactics in response to the German brutality. The aforementioned campaign of insurgents to cut telegraph and telephone lines directly influenced the outcome of the offensive, the destruction of railroads and bridges made the German heavy artillery incapable of supporting their men in mass while the French light 75mm guns were there at all times and gave them the advantage at the Marne. Basically German brutality in Belgium unified support in Britain and invigorated resistance in Belgium. How much so can not be measured obviously but that would be my big one.