The best way to handle this is to access the body of the message directly. The attached is modified to do that. Then the conversion of data to HTML and the need to faff around with signatures, when the signature was already in the message body before you started to process the message, is no longer required.
You can either write the content of the required cells directly or you can copy and paste it into a range, thereby leaving the signature in place. The multi-line cell needs no further processing when using this method, though you can treat it exactly as you would process a Word document from VBA
As your workbook didn't have the data range to be inserted (apart from the message you quoted) it was not possible to test the insertion of that, but it should be equally straightforward e.g. after the line
oRng.collapse 0 'collapse end
you could identify the range to be copied and paste it into the message range. This will insert the table as it is formatted in Excel. e.g.
Range("N2:O4").Copy
oRng.Paste