@Marco Thiel, amazing post as usual, and we excitingly adding it to Staff Picks! At some point you said:
I have failed to extract a list of all relevant spectral lines from Wolfram|Alpha
Did you mean the following code?
mercuryLINES = WolframAlpha["spectral lines mercury", {{"Lines:AtomicSpectrumData", 1}, "ComputableData"}];
TableForm[Rest[#[[1, 1]]], TableHeadings -> {None, First[#[[1, 1]]]}] &@mercuryLINES
You do not need to know the syntax of WolframAlpha function, just access it interactively (as explained here: Data Formats in Wolfram|Alpha):
