With respect to your questions on theosis and sanctification, et al, and your other siftings of this and that faith's truth claims, may I suggest another avenue of research as a way of testing those truth claims.
Find a few of the best exemplars of the faith in question from somewhere in the last two or three hundred years to the present day. Read their lives, preferably more than one account.
Then see how you answer the following questions.
1. is the faith believed by those from the earlier time the same as what that faith teaches and practices today.
2. is the life you see one that exists because of the particular faith held in common with others of the same faith, or is it incidental or in spite of that faith…insofar as you can tell?
3. Which lives seem to you most consistently a reduplication in kind of the best exemplars such as you see in the New Testament era and in the Apostolic Era?
If you know the answers to these questions, then you should have enough to tell if Lutheranism, Roman Catholocism, or Orthodoxy makes good on it's truth claims at an Apostolic level of fidelity.
Whichever faith you find produces holy ones of the same caliber as those you see in the New Testament, and have done so from their beginnings…that is the faith to follow.
That is what ultimately convinced me for Orthodoxy, not it's theology (which I found intriguing and profound), not it's antiquity (which was a point in its favor but not of itself determinative on the question of faithfulness), but rather it's saints, which it argues is the proof of the Church. I encountered the life of St. Seraphim of Sarov and was both destroyed and made alive by it. I knew he was the type of Christian I wanted to be, and that his kind did not exist in my theological world at the time, could not, though longed for. It was clear that if I wanted any shot at being remotely the kind of Christian he was it would have to be believing and following the same same faith as he did, because nothing…no path within my present faith ever got closer than the foothills of the faith and experience of God he knew. In months and years to come I discovered other lives very much like him all the way down to our own time, souls like Elder Porphyrios, Elder Paisios, Elder Cleopas, Mother Gavriella, St. John of Kronstadt, St. John of San Francisco and Shanghai, Fr. Arseny, St. Silouan, Elder Sophrony…blazing lives in Christ that could not be if Orthodoxy were not what it claimed to be. At least that is how I came to view their lives. The particular path they followed put them in opposition to Rome and to various forms of Protestantism, so…if those were true, then it would not follow that such depth of faith and relationship would be fruitful at the highest levels of Christian experience. If the others were truer…then even more and greater than these should be made manifest. But alas there are but few who even make it to the foothills…insofar as I am able to judge such things.
That is my experience and my interpretation of that experience. Yours may differ…but the test I think is valid…find the best exemplars of a given faith, and follow the ones who life and faith lead to the greatest depth of transformation in Christ. Find that and you are home.