It is true that they left after they received the entire explanation of the Real Presence, and that included the fact the Jesus is the Christ. But their initial objection was to the offer of His flesh in the literal sense; the explanation Jesus gave confirmed the literal sense.
Read the entire book of Romans, the entire book of Galatians, the entire New Testament and then compare James with that.
I have. The Church has. She wrote all these books. You are left to assert that James did not mean it when he said "by works a man is justified; and not by faith only", or Jesus in Mathew 25 does not mean it when He lists charitable work as the criterion by which sheep are separated from goats, or that Christ doies not mean it in Apocalipse when He said "I .. judge everyone according to his works", or again, Jesus did not mean to say, in John 6, "unless you eat the flesh of the Son of Man you do not have life in you". The references to works that are not salvific are works of obligation, or works of vanity, or works for reward, or works of Jewish ceremonial law. There is never a characterisation of works of love (=charity) as not salvific; to the contrary, love is set forth as the greates virtue and exhortations are made in every book to do them. The reference to the works being in plain evidence, as opposed to faith being an inward quality are meant to explain that one is impossible without the other; James never says that works are a mere outcome of faith, as the protestant spin suggests.
This assertion [that mass is a presentation across time of one sacrifice of Christ] is taught NOWHERE in scripture
"This is my body; do it in memorial of me" is in every synoptic gospel.
Then salvation is not a gift. It is a merit.
You do not die on the Cross to make the Eucharist possible. Christ did. To freely do what He asks is not a merit in the sense of commanding God to save you; the sovereignty of God does not suffer because of the salvific character of certain works. Is there a merit in acts of piety? -- definitely. But God remains sovereign through this, and your objection vanishes.