I have worked on algorithms (Perl, Java, PHP, JS) which processes CSV and other file formats, scrubs it, co-relate rows across CSV files and generate a new output file. One of my most credible project which heavily used JS+Perl is a Dashboard which describes tax collection across the nation, with drill-down, aggregation, achievement calculation, sorting, and access control for tax officers hierarchy. This project was developed 10 years back when Jquery/Angular were not a norm, and it is still used every day, and has a track record of ZERO bugs so far.
Assumption: You will provide (i) existing Google Apps Script and (ii) Sample data files as representative cases before project starts as part of acceptance testing.
I have studied the attached spreadsheet and clearly understood algorithm you have described.
Schedule:
D1: Put existing script under version control, setup a working environment and test that existing script works.
D1: Study current algorithm. Aim to fit new code within current algorithm, use similar variable/function naming convention, indentation style, etc.
D1: Write a pseudocode for updating additional columns within Sheet A based on data provided in Sheet 2 and CSV-Daten.
D1/D2: Implement pseudocode, test it with samples provided by you.
D3: Submit final code and patch as part of UAT.
D3: Fix bugs or assist in case of any issue while executing this application.
Feel free to contact me for any further clarifications, over Email/Skype/WApp.