You're correct that we can't prove the chain of custody for the Eucharistic miracles - this is discussed at the bottom of the article.
In general, there's no airtight "proof" for any miracle, and the best way to determine whether Christianity is true is to live the faith and see what happens. I wrote an article about this:
In general, there's no airtight "proof" for any miracle, and the best way to determine whether Christianity is true is to live the faith and see what happens. I wrote an article about this:
https://www.saintbeluga.org/the-rosary-the-ultimate-life-hac...