Method and knowledge in Plato's seventh letter

The philosophical excursus of the Seventh Letter (342a-345c) is the only passage in the whole Platonic corpus in which the philosopher speaks in first person about his method. If authentic, as I think, the letter provides us not only some fundamental biographical information about Plato, but also a description of his philosophy written for a different audience than the Akademia. The purpose of my paper is to analyze the excursus in philosophical terms, showing it to be consistent with what we can learn from the Dialogues. The excursus qualifies philosophy as a method of investigation different from all the other. This method requires some instruments that we must necessarily use if we want to get closer to the truth. The first two lines of the excursus are central in my analysis: ἔστιν τῶν ὄντων ἑκάστῳ, δι' ὧν τὴν ἐπιστήμην  νάγκη παραγίγνεσθαι, τρία. Some interpreters weaken the sense of  νάγκη, while others connect it with παραγίγνεσθαι: the first option negates that for Plato we necessarily use the τρία to get closer to the truth; the second option promotes the idea that, through the τρία, we necessarily reach the truth. Both options, according to me, are wrong. The right meaning of the sentence is that we must necessarily use the τρία to get closer (not to reach) the truth, as the use of παραγίγνεσθαι points out (παραγίγνομαι is indeed in the present tense, not aorist or perfect). In other words, Plato does not focus on the goal of the inquiry, but on its method. The excursus, in accordance with the Dialogues, does not suggest a strong epistemology, but a problematic one. This is confirmed by the fourth element, that is indicated by Plato with these words: τέταρτον δὲ ἐπιστήμη καὶ νοῦς  ληθής τε δόξα περὶ ταῦτ' ἐστίν, where περὶ ταῦτα refers to the τρία. Plato associates science, intellection and right opinion: does he want to deny the opposition between opinion and science, or to subvert the dialectic in its standard version? or is this an indicator of the fact that the letter is spurious? None of the above. Despite some authoritative scholars consider this passage problematic with respect to the Dialogues, it is actually consistent with them, as shown by Resp. 585b14-c1, Phil. 11b4-c1, 60d4-5, 66b5-c3, Leg. 688b2-4. The most considerable passages in the letter are those about the fallibility and the obscurity of the διαγωγή through the four elements (343b6-e1; 343), by which we can not solidly reach the knowledge of the fifth (i.e. the thing itself, the essence). To understand why the tools through which we must necessarily approach to knowledge are faulty by nature (πεφυκυῖα φαύλως) is the primary purpose of my paper and an inevitable investigation for anyone (considering authentic the letter) wants to get a sense of Plato's philosophy. Only by addressing this point it is possible to understand what kind of knowledge can be obtained through the method of inquiry called philosophy".