I will agree with everything you said, except about valentinian theology. According to Valentinian theology, Jesus brought the knowledge of God; not some "secret" as the detractors of Gnostic faith claim, but the firm belief, rightly or wrongly, that God can only be known by the words and actions of Jesus Christ.* Since Jesus Christ was unknown to those that came before, all that was written about God, from whatever source, must be erroneous, otherwise there was no need for Jesus to come at all. This theology was based on scripture, some of it recognized. As both Valentinious and Marcion were students of the Pauline schools of thought, they naturally favored this theology.
I would agree with Valentinian thought Jesus was teaching something new and that there was salvation in that teaching/gnosis. Marcion still looks like he is staying orthodox with the faith issue in the reconstructed texts but still gets accused by Church fathers of requiring belief in a particular understanding/gnosis of God for salvation.
The information being unknown and being the source of salvation was a problem because it not only excluded Abraham from the inheritance but everyone who was alive before Marcion or Val brought the truth to the world.
Vain, too, is [the effort of] Marcion and his followers when they [seek to] exclude Abraham from the inheritance, Irenaeus
I dont look at Marcion or Valentinious being a school of Pauline thought but all of them would fall under the umbrella of Greek philosophy but Im not what idea you consider particular to Paul.
Indeed heresies are themselves instigated by philosophy. From this source came the Æons, and I known not what infinite forms, and the trinity of man in the system of Valentinus, who was of Plato's school. From the same source came Marcion's better god, with all his tranquillity; he came of the Stoics. Then, again, the opinion that the soul dies is held by the Epicureans; while the denial of the restoration of the body is taken from the aggregate school of all the philosophers; also, when matter is made equal to God, then you have the teaching of Zeno; and when any doctrine is alleged touching a god of fire, then Heraclitus comes in. Tertullian
The two schools of thought, that Jesus fulfilled the law(James and gang), or Jesus replaced the law(Paul) were really at odds with each other for the first hundred and fifty years.
I think they were really at odds with each other within Judaism but outside amongst the Gentiles the Gnostic issue was going to pop up more because the gentiles were going to interpret what Jesus was doing from the perspective they are familiar with which was Greek philosophy. The philosopher-king was the ideal king and philosophy was seen as the source to an individuals salvation so they just interpreted Jesus as trying to teach what they thought was the key to salvation.
Jewish thinkers on the other hand didn't have philosophers as role models, like they did with the Law givers of their nation, so they were interpreting what Jesus was doing from that legalistic perspective. Catholics comes up with the faith concept leading to salvation, which may be invented by Paul because I havent found the idea before him.