1) This script illustrates how /fseek -l seems to always recount the lines from the beginning

That is intentional. The /fopen feature allows scripts to access and update files, possibly while these files are being accessed and updated by other applications. /fseek -l needs to count lines from the beginning every time to ensure it is taking account of all of the CR/LFs in the file. There are no plans to change this. If you want to count lines yourself, you can use /fseek -n and take the chance that none of the lines in the file have been updated since your last /fseek -n.