not sure exactly how to answer your question, "am i right?", but I could point some things out.
1. PV=nRT is the ideal gas law and requires ideal gasses for it to be accurate, I think you can substitute in some conversion factors to apply it to certain gasses under certain conditions, but as far as I know, a fuel air mixture is not an ideal gas and shouldn't be modelled as such.
2. P=F/A, ie. F=PA, therefore to increase F you can either increase the A or increase the P or both, but I'm not quite sure how this relates to jet engines.
3. I'm not sure how you come to the ramjet conclusion. Ramjets can use the shockwave generated by supersonic flow to create the compression required for efficient combustion instead of using a mechanical compressor, but I'm not 100% sure you need to be going supersonic to use one. You could design it to have subsonic flow at the inlet and then cause it to go supersonic by using applicable geometries, but then choking could affect you mass flow rate, and as astronuc said earlier, having a large mass flow rate is part of the thrust generation.
Hope that helps.