The concept of endogenous money is post-Keynesian, not Austrian.
The Austrian Free Bankers have the concepts of Inside Money and Outside Money.
There are some alleged affinities between Post-Keynesians and Austrians because they both criticize neoclassical general equilibrium.
But the Post-Keynesian conception of endogenous money is traceable to Hyman Minsky and by association, Joseph Schumpeter.
Here’s a commentary on the “debate” between Krugman, who is a New Keynesian, and Minsky, who is a Post-Keynesian.
In answer to your first question, not only is 100% reserve possible, but it was realized in history. For example, the Amsterdam banks of the 1600s were 100% reserve.
In answer to your second question, money substitutes are claims to money payable on demand at par. These are part of the money stock. In contrast, claims to be paid money in the future are credit claims. Normally credit claims do not function as a medium of exchange. Your buddy’s IOU would not be accepted by merchants as a medium of exchange, i.e., the most salable good in the market, under normal conditions. So, credit that comes from someone’s saving does not create money.