I gather, therefore, that you are using the full "Catholic/Orthodox" Bible, so that you have the complete Old Testament such as it was understood to be in the time of Jesus.
So, at least we are potentially working from the same, complete Scripture if we are going to be "Sola Scripturans".
Then our dispute is limited (thus far) to a narrow pair of issues:
(1) that the Catholic Church fell into apostasy and failed to follow the text of the Scriptures. What you said about the Scriptures that followed Isaiah has a basic truth to it: if some later tradition overtly refutes or attacks scripture, the later tradition is void according to Scripture. (This puts us in a pickle concerning Jesus' overturning of the explicit language of the Torah on the point of divorce, but we will let that pass for the moment).
Catholics (of the Roman, Eastern Rite or Orthodox variety), will of course tell you that the Catholic Church has not instituted traditions that contradict Scripture, and therefore the condemnations of Isaiah or Paul are not applicable. So our disagreement would repose on the content of the Catholic traditions, Eastern and Western, and not on the fact that they are traditions. Tradition is not bad unless it is bad tradition. Our dispute, then, is not over Scripture (in this case), but over Catholic practice.
Our other dispute would be over the adherence in a Sola Scriptura of your little church of students to the Scriptures itself. Have you ordained your bishops and presbyters by the laying on of hands (per Paul's letter to Timothy)? Do you baptize? Who does your baptisms? Do you share the bread and wine of the eucharist, per Jesus' admonitions? Do you permit your divorced members to remarry? Do you permit your little ones to approach the Lord and forbid them not? Do you confess your sins? Do you impart the Holy Spirit by the laying on of hands (what the Romans call "Confirmation", and I believe our Orthodox brothers and sisters call "Chrismation")?
What is your holy day?
And why?