How can a clock travelling at the speed of light look frozen in time to an observer, considering that exact time moment can only be reflected for one second & photons are destroyed once observed?
This is ultimately a trick question, in that the clock is not what is frozen in time, our observations of it are.
For this i would defer to the Relativity of simultaneity - Wikipedia
I should note that this is not a violation of causality, merely a naturally occurring lorentz transformation in observation which voids them from depicting a state of objective reality. Meaning if the seeking of truth is your goal, very little may be found in observation alone. Observation is essentially a perversion of the simplest truth on the topic there is. That all observation is local observation only. Meaning we may only ever see a representation of reality that has been altered due to the transitional nature of those particles. The question no physicist may answer is "How may a particle report on time passage rates of a frame it is not in?"
For instance, A, B and C in this digram occur in a different order based on the motion of the observer. The white line represents a plane of simultaneity being moved from the past to the future.
To understand this, then you need to understand whether the Clock would stop or just appear to stop.
The answer is that STR really cannot tell you. Lorentz version would tell you that it appears to stop whilst Einstein's version tells you it has stopped.
Lorentz is wrong because it would tend to stop.
Einstein is wrong because it does not correctly tell you why it stops.
It stops due to Energy considerations, but to understand that correctly, you need an absolute frame of reference. Einstein supposedly got rid of this. However, when it comes to GTR, he unwittingly reintroduces it, or at least he never admits that he intentionally did. Lagrangian Zero Potential Energy is an absolute reference frame.
So what would actually happen. Well, the clock could not exist at the Speed of Light but it can be assumed to be relativistic and it would actually slow down in its proper reference frame. However, an observer travelling with it would also slow down, so would notice no change.
No clock can travel at the speed of light; however, there are two kinds of particles which do travel at the speed of light, which we could call "clocks": the first is the photon itself, which is indeed "frozen in time" - for the photon, no time has passed between its emission wherever, and its observation somewhere else. The second is the neutrino. Now, interestingly, the neutrino actually has a very slight mass, which may be so small it is permissible under some quantum principle we still don't understand, and it travels at the speed of light. Now the neutrino actually does have some implicit sort of "clocking"mechanism - namely, it switches between tau, electron, and muon neutrinos on the average of every several thousand kilometers.
No matter, including clocks can travel at light speed. Nor can any observer.
Photos say photons are not destroyed. Merely absorbed, or reflected. Photons can come from an object moving at light speed but obviously not in the same direction because they are both moving at the same speed.
Time always exists, even for photons because they can red and blue shift and change frequency.
The clock stops because at the speed of light the matter in which the watch is made passes to another state of matter. You can tell it the fifth state of matter or the sixth state of matter. But this is actually the first state of matter.
Speed of light is just a threshold, not an absolute limit.
For instance, let make an analogy with the boiling threshold of water As for a gas (water vapor) we apply different physical principles, different from those for a liquid (water), and for this state of matter other physical principles are required.
Simply put, the observer is frozen in time too. That image is the only image in his mind for the entire journey. Time dilation doesn't only stop clocks.