it looks for me as it is just dangerous to lose his deep motivation. Why is that so by coming in contact with easy teaching, with logical understanding?
1. The state of "never-falling back" is attained on the eigth bhumi and the bodhissattva practices for three measureles kalpas (sutrayana).
2. The Buddha in the sutta pitaka also teaches that the qualities of people you are associating with will affect your own qualitities and therefore you should avoid the "unwise" which in the context of the Mahayana practitioner are self-centered and/or anti-Mahayanist people.
3. Living beings are innumerable and act improperly.
So to sumarize: endless duration of practice, bad influence of self-centered environment, limitless sentient beings to be liberated and their attitudes and behaviour towards the bodhisattva.
These are the reasons why a bodhisattva may lapse from the Mahayana and fall back to the Hinayana in the early and midterm stages of her/his carreer. Actually these are the reasons raised by all Hinayanists that are following the Theravada teachings (which is not Hinayana by nature) against the Mahayana. In most cases they hide their Hinayana motivation behind the argument "you have to help yourself before you can helf others". All Hinayanist share a fundamental incapacity to understand the function and workings of bodhicitta.
And Hanzze, investigate your own motivation why you as a non-Mahayanist are asking all those questions.
Are you interested in becoming a Mahayana practitioner or do you want to gain a neutral understanding of the Mahayana or do you want question the validity of the Mahayana?