Yes, only through Christ. But Christ was all over the OT and the righteous recognized Him. We know from John 6:46, and 1:18 that no one has seen the Father. However, there are several stories in the OT of people "seeing" God. So, Who could that be? Jesus, of course.
Here is a list of sightings: Hagar [Gen 16:7-13], Abraham [Gen 18-19:1], Jacob [Genesis 32:28--see Hosea 12:3-5], Moses [Exodus 33:11], Joshua [Joshua 5:13-6:2], Gideon [Judges 6:11-23], Manoah [Judges 13:2-22], Isaiah [Isaiah 6:5] Job [Job 42:5]. I think you are making the assumption that since no one had the NT in his hands in the OT, that they could not have believed in Christ. That just isn't so. Not only was there tons of prophecy about His Incarnation, He was actually very active in the OT. Jesus Himself says that the OT testifies about Him.
Great finds. The Angel of the Lord is Christ (I believe). Add to your list the scriptures about Melchizedek, which is from the NT but about an event in the OT:
Heb.7:1 For this Melchisedec, king of Salem, priest of the Most High God, who met Abraham returning from the slaughter of the kings, and blessed him;
2.To whom also Abraham gave a tenth part of all; first being by interpretation king of righteousness, and after that also king of Salem,, which is, King of peace;
3.Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God, abiding a priest continually.