Regarding space junk, I’m not sure. The fact that the spectrum had a set of the near IR atmospheric lines suggests it was doing a fair clip. All of the re-entry video’s and spectra I’ve seen have been relatively slow (and horrendously complex!) Whilst the most prominent metal lines are certainly Mg and Ca there are weak Fe features which are in the background of most spectra. No guarantee but I’d put my money on it being a tiny bit of something natural.