Why is it Chabad's minhag to put their head down when saying "L'david" when the Zohar says its dangerous to "fall on ones face" when reciting it?
According to the Shulchan Aruch HaRav 131:1
עַל פִּי הַקַּבָּלָה, יֵשׁ לוֹמַר מִזְמוֹר מ"ז, "אֵלֶיךָ ה' נַפְשִׁי אֶשָּׂא". אַךְ מִפְּנֵי שֶׁהָאוֹמְרוֹ וְלִבּוֹ רָחוֹק מִמֶּנּוּ גּוֹרֵם לוֹ לְהֵאָסֵף בְּלֹא עִתּוֹ, לְפִיכָךְ נִמְנְעוּ לְאָמְרוֹ בַּמְּדִינוֹת אֵלּוּ, אֶלָּא אוֹמְרִים "רַחוּם וְחַנּוּן כו' ה' אַל בְּאַפְּךָ כו'":
According to the Kabbalah (זהר ח"ג קכ, ב. הובא בב"י ד"ה כ' בספר הזוהר), one should say the psalm that begins, “[By David:] To You, O G‑d, I raise up my soul,” [as the supplication]. Nevertheless, because [the Zohar (ibid) teaches that] one who recites this psalm while his heart is in a distant place will die prematurely, [the custom] in these countries is to refrain from saying it. Instead, we say “Merciful and Gracious One,... G‑d, do not in Your anger....”
Note that the footnote adds:
"This is the custom in the Ashkenazic community. In his Siddur (and this is also the custom of Nussach Sephard), the Alter Rebbe prescribes that Psalm 25 should in fact be recited."
Either way, the problem, according to the Shulchan Aruch HaRav is not the actual putting one's head down but the recital without paying attention.
A quick peek at that Zohar: Seems that praying without intent in general is dangerous to one's wellbeing, though Psalm 25 is worse, since there it starts with "to You. O Lord do I pray" - and saying this mindlessly means one is flat-out lying.
So, no matter what one's prays one has to pay attention - thus, it seems, that Chabad saw no reason to single out this specific part of the prayers.