This technique of holding back the extract until the end of the boil is a fairly new concept that's caught on in the last few years. Here's some reasons why its a good idea in general:
Faster time from the start of the boil to the 1st hop
Less chance of a boil over
reactions of the extract (leading to lighter colored ...
I wouldn't risk piching your expired yeast for a kolsch, this is a style that should have very low, subtle to no yeast esters. Your old yeast may be still have a partial viable cell count, but it would produce too many growth esters for the style.
If this was a style that has strong ester profile (belgian, wiezen etc). I would pitch the old yeast in hopes ...
I would just keep it refrigerated. If your new yeast are to arrive in two or three days, it'll be faster than testing old one. And assuming old one is dead, keeping wort at fermentation temperature is far worse than keeping it refrigerated. It's already chilled, keep it that way.
What I would try to do, and I hope you did if it was possible, would be to ...
I would take some of the wort and make a starter with the yeast. If the yeast is good it should start fermenting sometime in the first 24 hours. If it doesn't I would throw it in the drain and wait for the new yeast to arrive.