Numbers in the OT were often rounded to even numbers (approximations). This is a convention, not a lack of knowledge of actual numbers by God.
The 144,000 is literal, but it may be more or less a few in the end. God will deal with the Jews until that number is reached. Just as He called Jeremiah, Paul, and others (but not unconditionally; they could have rejected their calling), so He can call out a select group, anoint them, set them apart in a different way than the many more Jews that would likely come to Christ during this period. This does not mean there will not be larger numbers believing from the tribes, just that these ones are raised up for a specific ministry.
Like Is. 46 and 48, the prophecy explicitly bases its fulfillment on God's ABILITY, not supposed exhaustive foreknowledge.
This is a valid question, but one proof text with a possible explanation (as well, many do see it as a more figurative number, even those who hold your omniscience view) does not negate scores of other verses and philosophical/logical discussions that would argue against the compatibility of free will and EDF (i.e. future partially unsettled, so not known as a certainty). Prophecy is a common objection against the Open View, but we do have a detailed response, depending on the specific prophetic passage. I would not get hung up on this especially since you really can't explain a mechanism for how God could know the unsettled future as settled (big logical/biblical issues with this).