Hello,
That gray rectangle doesn't appear using MS Word, it seems a LibreOffice/OpenOffice limitation, you can use   to avoid it:
<tr>
<td>2 Quisque ac augue eu nibh</td>
<td> </td>
</tr>
About the other question, Google Docs has many limitations when working with DOCX documents (for example adding charts and other complex contents) as it doesn't comply with the OOXML standard correctly. You'll need to mix paragraph margins/paddings and line breaks to get the needed output.
Regards.