Let me reduce the rocket example to two interacting electrons, A and B, traveling one after another, B in front of A. And there is an observer measuring their equal velocity.
The interaction takes place by sending photons to each other which travel at the speed of light. The speed of the photon is independent of its source speed. Nothing strange with that, same thing with water and sound pulses.
When the pair of electrons move relative the observer, the photon from A to B will need to traverse a longer distance compared to when the electrons are at rest. When they move at the speed of light the photon from A will never reach B and therefore B cannot be accelerated further.
Concerning mass/momentum increase one needs to interpret the concept of mass. Mass is a description of how much a force affects an object. The weaker motional response to a force the higher mass. So in this case, at the speed of light and no motional change, we would experience this as infinite mass/momentum.