That's fine, except that the Bible does not name the thieves, let alone give their backgrounds, and St. Dismas is a 4th century legend. If you can live with that as a matter of fact, more power to you. Might as well be pink unicorns on Jupiter. They are real if you are willing to believe they are.
It refers either way to the eternal life in the Heavenly Kingdom (the word “kingdom” IS used, mind you)
Yes the kingdom was mentioned but apparently Jesus (mis)understood it to mean paradise, which is neither heaven nor his kingdom, nor where he supposedly "went" that same day.
In either case, he put to paper what he learned from others, most likely, from the Blessed Virgin who he obviously had many conversations with
Where do you get that from?
No, but they still had names known to someone. To us the Good Thief is known as Dismas. Legends don't grow from nothing, so the reasonable way to name the man -- he is, after all, a saint and therefore a legitimate source of Christian names today -- is to follow the tradition. I recognize the possibility that he did in fact go by some other name that we will never know this side of heaven. There is a similar possibility that St. George did not use the princess's garter to tame the dragon, or that the Balaam's donkey did not talk. Still, the reasonable assumption is with the tradition, doctrinal or not, or in the donkey's case, with the Holy Scripture.
Where do you get that [St. Luke interviewing Mary] from?
From reason. He describes a lot of things in his first chapter that he could not know from anyone but Mary herself, and surely as a chronicler of the events in the life of the Church his primary source aside from Jesus Himself would be His Blessed Mother -- and she was available to the Early Church (Acts 1:14).
Now to the main topic, the hellenism of "paradise". I gave it some more thought and I now think that while the two hypotheses I advanced earlier: (1) that it is an inadvertent hellenism by St. Luke and (2) that Jesus used Greek and used the common word because Dismas was Greek, -- are valid as hypotheses, there is a much simpler explanation that does not involve any imprecision in speech. Here it is.
St. Dismas was not Jesus' disciple; he was not even, most likely, an educated man. His wordlview clerly contained some understanding of sin, -- since he spoke of it, -- but not much more. Since he lived in Jewish lands, let us make a simple assumption that he knew a little bit of the story of the Fall. Unlike the assumption (2) that he only understood Greek, that is a natural assumption to make, as the story of the Fall only requires a rudimentary knowledge of the Jewish faith. Well, now Jesus has a difficult under the circumstance task, to convey to the dying man that his sin has been forgiven and that he gained everlasting life for his just completed work of charity and faith. Ordinarily, it took Jesus parabolic discourses to speak of the Heavenly Kingdom. This was not an option at the time. So instead, Christ reduced the promise of heaven to the simple thing the Thief did know: you will be with me in a state of supernatural happiness, like your parents Adam and Eve prior to the Fall. for that, there is a word. The word is "Paradise". So Jesus, the master teacher, taught as he always does, with precision: Today you will be with me in Paradise.