PsiCop

What we know of Marcion suggests that he was personally a bit more spiteful than most others. Of course, the historical record on him is littered with the claims of his foes ... but it can be safely said that he personally offended a number of people during his career, and that at least some of his teachings were a contrarian reaction to those of others whom he disliked.

A good number of scholars are now treating Marcionism as distinct from Gnosticism, of which Catharism was a medieval descendant (a couple generations removed). Marcion did not teach a separate, "secret" esoteric doctrine, and it appears he did not have an initiation process any more involved than that of other Christians, i.e. it was baptism (other Gnostic sects had some complex initiation schemes and multiple, layered rites to accomplish them). Marcion also emphasized the power of scripture in determining doctrine whereas for Gnostics, personal experience (or gnosis) of the Divine was paramount.

On these scores Marcionism is, indeed, different from Gnosticism. However, like you I consider them more related than separate; philosophically Marcionism espoused dualism which could be just as pronounced as any Gnostic sect. Marcion's vehement condemnation of the Hebrew god YHWH was also a part of other Gnostic sects. Marcion's morality was also largely Platonic, which again connected it with other Gnostic sects. In many ways one could say the basic worldview of a Marcionist was much more similar to that of another Gnostic, than to other "orthodox" Christians.

There is also a possible historiographical connection between the Cathars and the Marcionites. Marcionism began in Anatolia (its founder was from Sinope on the Black Sea coast, and well-known throughout the Pontus province, even before he emerged as the leader of a new church and moved to Rome to propagate it). Anatolia remained the "seat" of Marcionism after him.

The Cathars, meanwhile, descended from the Bogomils, who in turn descended from the Paulicians. The Paulician sect was the Anatolian remnant of older Gnostic sects, and may very well have included some descendants of Marcionism as well.

While you are correct that Cathars cited "orthodox" scripture this way, one must take care not to go too far with this. For Cathars, as with other Gnostics, the "ultimate" doctrinal authority was not scripture, it was, rather, personal experience of the Divine. Yes, the Cathars had scripture of their own, particularly The Gospel of the Secret Supper and the Book of the Two Principles (unfortunately the text of neither is online, since they are reconstructed documents and remain in the hands of academics at the moment). These works merely exemplified their beliefs, and did not define them. Much of what they taught was conveyed in person, and medieval Cathars themselves often claimed that the only way to "know" their beliefs would be to undergo consolamentum (i.e., to convert and become initiated). Backing this up is the fact that the spiritual authority of the perfecti/parfais lay in their having gone through consolamentum, rather than from having studied writings or being able to recite doctrine. Initiation, for Cathars, was the gateway that opened up "knowledge" of God, and that was what they considered most important.

The Cathars who cited scripture in the way you cite, were really trying to show that they weren't all that divergent from the Catholics. In other words, they were using passages like the above apologetically, rather than authoritatively. It was their use of such apologetics that convinced pro-Catholic missionaries like St Dominic Guzman that ultimately the Cathars could not be persuaded out of their heresy. That determination, once it was adopted by the whole Church itself, led to the Inquisitions.

Outside of movements such as Marcionism and most types of Gnosticism, recognition that the OT and NT gods are two different beings, is largely a modern phenomenon. The reasons for this are complex. In at least some cases it was motivated by contrarianism ... i.e. people like Tertullian argued along these lines:

1. Marcion said the OT and NT gods are different
2. Marcion is always wrong because he's a heretic
3. Therefore the OT and NT gods must be the same being

Yes, it's irrational and fallacious reasoning, but it's powerful. It's partly because people like Marcion taught duality, that "orthodox" Christians dug their heels in even harder on the issue and adopted unity instead.