Ps. 110:1 contains a Messianic use of the word "Lord" (ADHON) and
this passage clearly distinguishes between THE LORD (YHWH) and the Lord of David (the Messiah). Furthermore, ADHON is used with reference to the Pharaoh of Egypt (Gen. 40:1) and also wielded for Joseph who was sent to Egypt by God in order to preserve his family (Gen. 42:10, 30). Cf. Ruth 2:13; 1 Sam. 1:15. So Thomas' use of KURIOS in Jn. 20:28 need not indicate that he was identifying Jesus with YHWH.
As for the term ELOHIM, it too is a term that can be applied to men, angels, and God/gods. Isa. 9:6 probably calls the Messiah, EL GIBBOR. The Catholic NAB, when commenting on this passage, makes the following observation:
"Upon his shoulder dominion rests: authority. Wonder-Counselor: remarkable for his wisdom and prudence. God-hero: a warrior and a defender of his people, like God himself."
Please note the prophecy in Zech. 12:8 as well and its use of ELOHIM.
Bob Utley makes these remarks on 12:8:
"the house of David will be like God, like the angel of the Lord before them" This is a striking metaphor used in the sense of God's empowering of His people. The term for God is the term Elohim, which is used in the sense of supernatural beings (cf. Exod. 4:16; 7:1; I Sam. 28:13; Ps. 8:5; 82:1,6).
The angel of the LORD is often seen as God's representative among the people (cf. Exod. 13:21; 14:19; 23:20-21; 32:34; 33:2,14-15,22). In two passages David is likened to the angel of the Lord (cf. I Sam. 29:9; II Sam. 14:17,20; 19:27). Remember there are three phrases (no VERBS) here which build on each other for literary, not theological, effect.