I found the solution to the problem of excel opening in read only mode. It was obvious, as are many solutions AFTER you have found them. It was also written in my first post: the table is linked to the access app, therefore the workbook is to be considered open. When I try to open it again with workbooks.open, excel can only open it in read-only mode. One may complain "why didn't excel say so, that the file was already open"? This I don't know, and would like to know.
Anyhow, I replaced

DoCmd.TransferSpreadsheet acLink,...
with
DoCmd.TransferSpreadsheet acImport, ...

and now the .xlsm file opens correctly in read/write. Thank you everybody for your attention and interest. Now, if I or you could find the reason why xlValues is not recognized by the compiler, even though
it is correctly listed in the object browser...