Addressing the Greek, strictly speaking there does not seem to be a strong distinction between προσκυνέω as it relates to God and as it relates to men. Exodus 20:5 (LXX) explicitly states that one shall not bow down before (οὐ προσκυνήσεις) other gods, but, as you point out, the action is seen before men in the Old Testament. Other examples are:
1 Chronicles 29:20 LXX
And they bowed the knee and worshipped the Lord, and did obeisance to the king [προσεκύνησαν τῷ κυρίῳ καὶ τῷ βασιλεῖ].
3 Kingdoms (1 Kings) 2:13 LXX
And Adonias the son of Aggith came in to Bersabee the mother of Solomon, and did obeisance to her [καὶ προσεκύνησεν αὐτῇ].
On the other hand, the word λατρεύω (which also appears in Exodus 20:5) applies in the Old Testament exclusively to the worship of God (or other gods):
Exodus 3:12 LXX
Ye shall serve God upon this mountain
Deuteronomy 28:46-47 LXX
And these things shall be signs in thee, and wonders among thy seed for ever; because thou didst not serve the Lord thy God with gladness
and a good heart, because of the abundance of all things.
Daniel 3:18 LXX
Be it known to thee, O king, that we will not serve thy gods
3 Maccabees 6:6
When the three friends in the land of Babylon of their own will exposed their lives to the fire rather than serve vain things, thou
didst send a dewy coolness through the fiery furnace.
As an outcome of the iconoclast heresy in the 8th century, the Seventh Ecumenical Council clarified the theological use of the two terms pretty much within the scopes found in the above Septuagint examples.