Also, what happens if (when) Greece defaults? Do the T-bills become worthless, or are they considered a different kind of debt than other national debts?
They aren't actually paying that. Since the EU/ECB/IMF gave them all that money they have a nest egg and don't have to go to the market and sell more bonds.
AFAIK the 26% is from secondary sales, people who bought a bond before and now, in order to sell it on, have to offer a high rate.
However if the EU/ECB/IMF money runs dry, and they need to go back to the bond market, then this is the rate they would pay. As you said, that's impossible. So if the EU/ECB/IMF money runs dry, they'll default.
even if you could directly invest in greek bonds, you probably would not want to do this...
http://www.pdma.gr/(S(fcxze5rfyjyze2555hw43555))/ODDHX/Stati...
Treasury bonds are actually really easy for a retail investor to buy.