The reality is electrons have a probabilistic nature, which is born of their wave-like nature. There is no transitory period between energy level transitions.
Your case about multiple electrons decaying simultaneously to the same spot is interesting. Your best resource is the Pauli exclusion principle, where two electrons have can't occupy the same state and spin. But the idea that one transition interrupts the other by "beating it " in a race to the lower energy level is looking too far into things because in an X Ray tube K and L transitions are happening all the time and their intensities are based on probability but with a VERY high sample size.
So I don't think that this event, if it exists, would make any significant affect on a characteristic spectrum. Understanding wavefunctions may help eliminate your concern over this.
Hope this was helpful. I'm a graduate student studying this stuff but am in no way an expert haha
This question is ripe with classicalities so it's best to indirectly answer the question by explaining how we talk about QM.
First: we do not talk about trajectories in QM. Saying the particle had a trajectory that would allow us to describe a transition being interrupted is intractable in the language of QM. We have a couple of analogous concepts such as time uncertainty and lifetime of the transition but these are distinct from the duration of the transition.
Second: if two distinct but identical particles were to 'compete' to decay into one lower state we cannot talk about which particle actually did decay. The two particle excited state is a superposition state where both particles have probability to be in either state but we cannot know which is which, there is no way to label them. More exactly, any label we apply to the system gives us an observable that would disturb the system and invalidate what we were attempting to measure in the first place.
For the issue of the X-ray tube I think you are concerned about a cascaded decay. This can happen in a few ways in atomic systems. For a cascaded decay the electron decays to a lower unstable or metastable state then again to (likely) the ground state. This will produce two photons that will likely be of different wavelengths and hence distinguishable in a fine grained measurements. And potentially a measurable time delay depending on the lifetime of the intermediate state.
In other systems, particularly crystals such as KTP and BBO you can have nonlinear optical effects but the one we're interested in here is uniquely quantum and that's Spontaneous Parametric Down conversion or SPDC. Here you send in a laser of frequency 2f and get out two identical (and possibly entangled) photons each of frequency f to some uncertainty.
Of course none of the last two paragraphs matter because your second point doesn't conserve energy but it should give you perspective. Those decay paths produce photons of different energies and are distinguishable so you don't have to worry about interference assuming you measure the photons energies.