I'm curious as to how one corrects for uneven distribution of dust as such between the observer and observed.
Very tough problem. Solving it was one of the 20th century's greatest scientific achievements. It took a deep understanding of atomic and molecular spectra, a deep understanding of the composition of the interstellar and intergalactic media, and some sophisticated instrumentation.
The basic idea is this: find the strongest absorption line, and that represents the Lyman alpha transition of hydrogen from the the densest intervening cloud. Once you have that, you know where to find the absorption lines for all the different elements and molecules likely to be found in that cloud. The relative strengths of that series of absorption lines allows you to measure the composition of that particular cloud.
Once you have found all of those lines, and corrected for them, the strongest remaining line is judged to be the Lyman alpha line of some other cloud. It's not lying at exactly the same frequency as the first Lyman alpha line, because it's at a different redshift. Find all the lines from that cloud, and correct for them. Repeat the process until all the remaining lines are consistent with coming from the source object.
The multitude of absorption lines arising from the Lyman alpha transition is known as the Lyman Alpha Forest.