You continue to evade the primary objection, and now repeat the second one, seeking to portray Rome as being in unity while Protestants I referred to are ever morphing in core doctrines. However, this has already been essentially been shown to be deceptive misleading.
In response to your 1st question, a core truth for those who in practice hold to the supremacy of Scripture as the infallible word of God - which was a core principal that defined Protestantism, and which is contrary to your liberal application of the term - has been one that, as stated before has demonstrable Scriptural substantiation. Therefore the core truths that all of the above class of Protestant denominations most universally have historically held to are the one they hold to today, which again, are the truths articulated in Nicene Creed along with the supremacy of Scripture and salvation by grace. In addition, multiple surveys consistently show that members of these churches have more unity around critical faith and moral issues than Catholics.
An examination of statements of faith of major evangelical denominations (S. Baptist, Assemblies of God, etc.) reflects this unity, and thus they are known for disfellowshiping teachers who depart from these tenets, while deviations from these core truths are most typically the domain of those who follow a man or office which effectively carries more authority than the Scriptures themselves.
As for disagreement outside these most universally held core issues, these are allowed to a limited degree among teachers, as Rome also allows, and despite its infallible office and teachings, its members evidence much disagreement among themselves.
As for how do i am to know that i am right and those that disagree with me are wrong, the answer has already been given, which is by the Berean method, which you reject, that of manifest Scriptural attestation to doctrine, using exegesis is manifests (contrary to Rome, which relegates historical events to being fables) and which is also affirmed by the power of God. It is abundantly manifest that both Jesus and the apostles invoked the authority of the Scriptures, and appealed to the reason of men. ( Mt. 22:29-45; Mk. 12:20; Lk. 12:57; 24:25-29; Jn. 5:36-46; Acts 17:2; 18:28; 28:23; 2Cor. 4:2 Rm. 3,4; 2Pt. 1:16-21) And rather than the church declaring that it was infallible whenever it spoke according to a declarative formula it infallibly declared, it propagated doctrine that had the testimony of Scripture and the power of God. (Rm. 15:19; 2Cor. 6:1-10)
Now the questions for you are,
1. On what basis am i to know that Rome is infallible?
2. How many verses of Scripture has Rome infallibly defined?
3. Can you provide a complete infallible list of all infallible teachings?
4. Do all RC priests and laity agree with the teachings of Rome?
5. Do Catholics evidence greater unity in moral views and commitment than those in churches who hold to the supremacy of Scripture as the wholly inspired word of God and salvation by grace?
6. What is more beneficial: unity based upon implicit trust in an office which declares itself infallible, and by which it has had people killed who disagree with her, or unity in the gospel truth that results in demonstrable effects of regeneration, even if they can disagree somewhat in other issues?
7. Is the use of physical violence by the church to discipline church members, or subdue theological enemies, Scriptural?
8. Is praying to Mary and departed saints found in Scripture?
9. What transcendent authority did the apostles appeal to in preaching that Jesus was the Christ?
10. Is any other objective authority declared to be 100% inspired by God in whatever it says?
11. Were the Bereans wrong in examining the preaching of the apostles by the Scriptures?
12. Were the Pharisees wrong in teaching a doctrine as from God that went against Scripture?
You live in a drean world if you think that Protestantism is united in anything. The millions of sects is manifest. and morphing doctrines are manifest, while Catholicism has lasted from Christ's time till today, unchanged by the thousands of heresies that tried to shake it apart. It's all there for anyone to see, in space (Japan, Paris, or Montana), in time (in 325 A.D. or 2010), same religion, Catholic, universal.
A study of history will bring you back to St. Peter from Benedict XVI.
Then read what Christ promised to St. Peter.
God bless.