At the time, the deck (900 sq ft) needed to be stained and sealed.

The building I managed had one of these, and it took me a year to convince the owner to let me seal it shut - and then the angels sang! 

Give this a go.... blows and home sleepo have a drain king.... its a pressure balloon that attaches to a hose and expands in the pipe to seal it from coming out the back end where you are sitting.Find the distribution box.

Some surfaces may need to be covered or sealed.

The 2 story though you could hear yelling and doors slamming as the lower floors are not sealed off.  

here are a few options: -check that all wall penetrations and gaps have been properly sealed, including outlets and other electrical boxes.  

You could provide both with the addition, then seal off the interior connection and only use the additional entry when converted.

Replacing tenants would cost more than sealing the basement for a little rain water or snow melting in my opinion.

I think I'll try cleaning it once more with one of the items suggested at the link Jay provided and then seal the driveway with a topcoat.

I have stained/sealed concrete in my newest rental and it's held up pretty well so far.