I’m sure you are right about the provenance of the title ‘lord’ for Jesus. Part of the problem here is that we have very little direct evidence for how Paul interpreted and responded to the language of paganism.

The verb harpazō and related terms seem mostly to be used in the LXX in the sense of ‘seizing violently’ - there are numerous references to lions ‘seizing’ their prey; it is also used for robbery, theft, plunder. Gordon Fee in his commentary thinks it cannot have this sense here, but if we think of it as a word that actually describes the hubris and blasphemy of Caesar rather than something that Jesus might really have done, I don’t see that there is a problem.