Thanks for that detail Leolaia about no mention at all of 2 Corithians appearing in Clement's material. Its hard to explain his never mentioning parts of it if he really were the source behind Paul's letters. So scratch that scenario.
I really appreciate all the facts you've brought thus far. Don't burden yourself with too much about other stuff about the supposed clementine authorship. Its simply fun for me considering speculations like that and others like Paul being a Herodian.
Back on topic, its true that a person can alter their style depending on the genre and also how they develop over time. But Hebrews isn't such a different genre of literature (like poetry or apocalypses say), that we should expect a vastly different style of expression.
The Pauline letters generally have a basic similar form to them. The way the material is expressed is more refined in Hebrews, so the question is did Paul really become a more improved writer from his earlier material in the 50s? Who knows.
Origen, said one could never know who wrote Hebrews but he really believed the ideas were Paul's. Well he said only God knows who wrote it, but I'm going to being obstinate and not acknowledge any deities....in one of those moods