Why did Samson the holiest ever person at birth 'degrade' himself and 'marry' a philistine woman and how did she cause his 'downfall? What made him tell her his 'precious' secret? Did that he came from the Dan tribe have to do with it?
R. Gershon Weiss a"h discusses this at length in his book Shimshon's Struggle. For one thing, he says, it's not at all certain that Delilah was a Philistine by birth; she may have been a born Jewish woman. In fact, he suggests that the wording of Rambam in Hilchos Issurei Biah 13:16 (וכן שמשון גייר ונשא, "and similarly Shimshon converted and married") suggests that only his first wife, the girl from Timnasah (Judges ch. 14), was a convert. (Although Touger there translates the phrase as "Samson converted [women] and married [them]," in the plural.)
He goes on to note the rule in the Midrash (Ruth Rabbah 4:3) that when a person is introduced with the formula ושמו\ושמה פלוני, "his/her name was X," then it indicates that they're righteous. Delilah is introduced that way.
Also, he points out that the verses don't describe her as beautiful, making it unlikely that he was attracted to her looks.
The gist of his explanation, then, is that she was indeed originally righteous (and indeed must have been, to risk marrying an aging fugitive from Philistine justice - remember that by this time he had already served as shofet for 20 years). That's what attracted Shimshon to her. What happened, though, is that the Philistine leaders offered her money to betray Shimshon, while implying that they'd spare his life if they captured him (in Judges 16:5 they say only that they plan to "overpower him, tie him up, and make him helpless"). The lure of money warped her judgment and caused her to rationalize that by handing Shimshon over to them she'd actually be saving his life - and then, as the Gemara goes on to say there, she sank so far down that she ended up committing other serious sins, all the way up to and including adultery.
It is clear from the Gemarah that you mention in Sotah as will be discussed below, that he fell in love with her. I won't discuss how a tzaddik could marry such a person because it is not for us to make ta'anos (claims) on such a holy person. However, as far as the second part of your question as to how Delilah brought about his downfall, we can explain as follows:
The Gemara in Sotah 9b states:
The verse states: “And it came to pass afterward, that he loved a woman in the valley of Sorek, whose name was Delilah” (Judges 16:4). It is taught in a baraita that Rabbi Yehuda HaNasi says: Even if she had not been called by the name Delilah, it would have been fitting that she be called Delilah, for she weakened [dildela] his strength, she weakened his heart, and she weakened his deeds, thereby decreasing his merits. (Sefaria translation and notation)
So it would seem that Delilah had a strong influence on him that proved detrimental to Shimson's physical, emotional and spiritual faculties.
Interestingly, the Ben Yehoyada writes:
דִילְדְּלָה אֶת כֹּחוֹ, דִּילְדְּלָה אֶת לִבּוֹ, דִּילְדְּלָה אֶת מַעֲשָׂיו (שופטים טז, יח). פירש הרב עיון יעקב ז"ל לשון דלות ועוני. ונראה טעם בזה כיון שהלך אחר עין שלו בדרך הפכפך הפך הדין נהפכו לו אותיות עין לאותיות עני שנעשה עני ודל בשלשה אלה.
ונקיט ג' דברים מַעֲשָׂיו כֹּחוֹ לִבּוֹ דראשי תיבות שלהם 'מלך' כי הוא היה שופט שהוא במקום מלך באותו זמן והיא דלדלה אותו בשלשה דברים הרמוזים בשם המעלה והשררה שלו וזה היה לו בשלשה דברים הנזכר מפני שהלך אחר מראה עיניו שיש בהם שלשה גוונים לבן וירוק ושחור. ובני ידידי כבוד הרב יעקב נר"ו פירש בשביל שזאת היתה אשה שלישית לכך דלדלה אותה בשלשה עד כאן דבריו נר"ו.
She weakened (דִילְדְּלָה - 'Dildelah') his strength, she weakened his heart and she weakened his deeds - The Rav explain in Iyun Yaakov that (the term 'dildelah') is an expression of making poor and impoverished. And it appears that the reason is because since he followed after his eye - 'ayin' in a (literal. upside down) fickle way, the judgement of his was also equally turned upside down. Thus the letters of "עין" ("eye") can be rearranged / turned upside down to read "עני" ("poor"), since she made him in poor in these areas.
And it mentions three things (that she weakened him in)- "מַעֲשָׂיו כֹּחוֹ לִבּוֹ" - "his deeds, his strength and his heart", and the roshei teivos (the first letter of each word) can be arranged to spell "מלך" - "King", and his role was that of judge which was in place of a king at that time, and she weakened him in three areas where he lost his sense of virtue and dominion. And it was to him in these three things that we mentioned because he went after the sight of his eyes which have three shades - white and green and black.
So it would be fair to say that Delilah was quite a wily personality that Shimshon was besotted with, and she was able to systematically strip him away of these three previously, strong character defining traits.