I have to extract cell values into a summary spreadsheet from multiple closed workbooks saved in Google Drive.

I'm familiar with Laurent Longre's INDIRECT.EXT, but run into a problem while trying to use INDIRECT.EXT inside an INDIRECT expression that combines various worksheet paths and names from the their lists in an open summary worksheet. I suspect the problem may be in the 'quote the quotes' mess in the concatenated expression, but can't find anything wrong in the formula.

My question: is the combination of these two functions, i.e., INDIRECT.EXT inside INDIRECT, allowed? I do know cell ranges and defined names cannot be used inside INDIRECT.EXT.