Photons are elementary particles, part of the SM, they are traveling at speed c in vacuum, when measured locally.
The world line (or worldline) of an object is the path that object traces in 4-dimensional spacetime. It is an important concept in modern physics, and particularly theoretical physics.
Now photons do move along lightlike worldlines. You are correct, because on the lightlike worldline of a photon, the spacetime distance is 0. The photon does not have a reference frame, so it does not make sense to say " what would it look like from the photon's frame". But, since the spacetime distance for the photon is 0, you are correct, saying that the emission and absorption are casually connected for the photon and we could say the photon experiences both emission and absorption in one.
Now you are saying that altering the photon in the future (on the fly) would alter the photon back in time to the source.
In reality, you can use the double slit experiment with a detector, positioned after the slits, to show how there will be no interference pattern because the detector interacted with the photons after passing through the slits.
In this case the wavefunction of the photons is collapsed (after passing through the slits) and the phases of the photons are lost, and there will be no interference pattern.
When the electron suffers inelastic scattering, it is localized; this means that its wavefunction collapses and after the measurement act, it propagates roughly as a spherical wave from the region of interaction, with no phase relation at all with other elastically or inelastically scattered electrons,” Frabboni said. “The experimental results show electrons through two slits (so two bright lines in the image when elastic and inelastic scattered electrons are collected) with negligible interference effects in the one-slit Fraunhofer diffraction pattern formed with elastic electrons.
The example is for electrons but works the same for photons. In the example there is a crystal positioned after the slits, and this is inelastic scattering which means the photons interact with the detector, thus there will be no interference pattern.
You could argue that this is like changing the photon back in time, but in reality this is because of QM and the wavefunction is set up so that interacting with the photons collapses the wavefunction and the phases are lost and there will be no interference pattern. You could say that photons do not experience time or that they experience the whole timescale between emission and absorption in one, but photons do not have a reference frame.