I see that Import/Export only works if the path and filename is exact. Is this the intention?
Yes, is the only reliable way of matching song in Jaikoz and spreadsheet.
Would it be better to export filename without the path info? Or import based on MBID?
No, because you have multiple files with the same name in different folders, you'll almost certainly have some files without an mbid , and maybe have multiple files with the same mbid so I don't understand how this would work.
Also, is there a plan to support XLSX files?
No I cannot see much point in me spending effort on supporting multiple spreadsheet formats, xls (and csv) are well understood by most spreadsheet software and you can always convert to xlsx format within the spreadsheet if you really want to.