Connection Issues with Informix OLEDB Provider
So, after giving up searching for answers, I started trying every imaginable combination of properties with different registries set and not set and I managed to get it working.

The Connection String was missing UNICODE=TRUE. 

I can't say for sure, if I already tried this before and the error originally came from incorrectly set registry entries or if that simple solution slipped past my testing. 

Anyways here is what I did, in addition to what was mentioned in my original post (no idea if that made any difference):


registered the ifxoledbc.dll (32Bit and 64Bit) with the respective regsvr32.exe (Originally I used the 64-Bit regsvr32.exe for both)
Added registry entries in Computer\HKEY_CLASSES_ROOT\Ifxoledbc\CLSID (64Bit) and Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Ifxoledbc\CLSID (32Bit) with name: OLEDB_SERVICES type: REG_DWORD and value: 0xffffffff


Concluding:

I'm still surprised, that the RowsetViewer somehow bypassed the need for the UNICODE property. Maybe it automatically sets it to true?  

The interface:unknown error I mentioned with the viewer seems to have been fixed in a newer version of the Informix Client SDK (Only happened with 64-Bit Operating Systems using 32Bit Tools).

During my successful testing I also noticed, that Excel seemingly didn't need the %INFORMIXDIR% environment variable. FYI, when I was testing the ODBC datasources i had to edit the %INFORMIXDIR" to either point to my 32Bit or 64Bit folder (Not doing so resulted in a unable to load shared library error). The 64Bit datasource was also incompatible with my 32Bit Excel. With the OLEDB Provider however, it connects just fine with %INFORMIXDIR% still pointing to the 64Bit folder.