Since you are using the WSP approach to module development, there should not be a problem creating the module in DNN 4.9 and installing it in a 4.5.3 site PROVIDED that you have not used any core methods in your module that were added to the DNN framework since DNN 4.5.3. At anyrate, such problems are usually not noticed during the module install - only later when the module is actually added to a page and used.
Did you check the web.config of the company site ONTO WHICH you are attempting to install to verify that there is no "xlmns=" attribute in the top level <configuration> node? If that is the cause of the problem, it would be with the company website, not your own which you used to develop the module.
Likewise, did you check that the root folder and web.config file of the company site ONTO WHICH you are attempting to install gives full permissions to the ASP.NET worker process (NETWORK SERVICE account, etc.)? Even if you have been able to install other commercial modules to this site, unless those installs needed to write to the web.config file (as your module install must do to add a directoryName to the <codeSubDirectories> node), the problem might not have shown itself previously.
Could you please post the entire <codeSubDirectories> node of the company site onto which you are installing? The one from your development site:
<!-- register your app_code subfolders to generate granular assemblies during compilation
<codeSubDirectories>
<add directoryName="sub-directory name"/>
</codeSubDirectories>
-->
<codeSubDirectories>
<add directoryName="HTML"/>
<add directoryName="TaskList"/>
</codeSubDirectories>
appears to be correct and does show the addition of the directorName for TaskList.