Psalm 133 - and its grammar

This is an odd presentation of Psalm 133. It happened sort of by accident. Green is for grammar.

A song of the steps of David See how fine and how pleasing it is that brothers live as one

שִׁיר הַמַּעֲלוֹת לְדָוִד הִנֵּה מַה טּוֹב וּמַה נָּעִים שֶׁבֶת אַחִים גַּם יָחַד

like the finest oil on the head dribbling on the beard the beard of Aaron dribbling to the mouth of his clothes

כַּשֶּׁמֶן הַטּוֹב עַל הָרֹאשׁ יֹרֵד עַל הַזָּקָן זְקַן אַהֲרֹן שֶׁיֹּרֵד עַל פִּי מִדּוֹתָיו

like the dew of Hermon dribbling on the mountains of Zion for there יְהוָה constituted the blessing life for ever

כְּטַל חֶרְמוֹן שֶׁיֹּרֵד עַל הַרְרֵי צִיּוֹן כִּי שָׁם צִוָּה יְהוָה אֶת הַבְּרָכָה חַיִּים עַד הָעוֹלָם

שִׁיר	A song of	construct
הַמַּעֲלוֹת	the steps	feminine plural
לְדָוִד	of David	proper name, David with preposition
הִנֵּה מַה טּוֹב	See how good	adverb - no agreement in gender or number
וּמַה נָּעִים	and how pleasant	adverb - again no agreement in gender or number
שֶׁבֶת	to live	infinitive in construct, weak root ישב
אַחִים גַּם יָחַד	brothers as one	masculine plural
כַּשֶּׁמֶן	as oil	- is this construct?
הַטּוֹב	the good
עַל הָרֹאשׁ	on the head	definite head - almost like English generic definite!
יֹרֵד עַל הַזָּקָן	running down on the beard	yored - the sound of brooks running - or singing! like shir which this psalm is, definite beard
זְקַן אַהֲרֹן	the beard of Aaron	specific priestly beard
שֶׁיֹּרֵד	that is running	the grammatical use of shin
עַל פִּי	to the mouth of	construct
מִדּוֹתָיו	his clothes	plural with possessive pronoun third person masculine singular
כְּטַל חֶרְמוֹן	as dew of Hermon	construct
שֶׁיֹּרֵד	that is running	repetition
עַל הַרְרֵי	to the mountains of	construct plural
צִיּוֹן	Zion
כִּי שָׁם	for there
צִוָּה	commanded	third person piel (this verb is always piel - intensive) singular. You can't see the piel from the consonants alone. So was the piel made up later than the written texts?
יְהוָה	the LORD
אֶת הַבְּרָכָה	+ the blessing	a definite blessing
חַיִּים	life	life - plural!
עַד הָעוֹלָם	to the age	another definite age - life to eternity