Jane agrees, and they agree to a pay-by date, and Jane wants 10g of gold as payment for the loan.
How did Joe agree to an impossible method of payment? They both know there is no gold to be gotten. So we must conclude that either Joe knows he can work for someone and get paid off in 10g of gold [or make money some other way] and thus be able to give jane her interest, or else, if he suspected people would be hoarding their gold and he would not be able to get any in time to pay off Jane, he would stipulate that he be able to pay the interest in some agreed upon thing that is worth 10g of gold.
Now where does this extra 10g of gold come from in a 100% reserve system?
It drops down from the moon. Just kidding. It isn’t needed, as explained above.
But that 10g has to come from somewhere, right?
It has to come into Joe’s posession, yes. And he gets it by working for someone for 10g of gold, or making money some other way. And if everyone is hoarding gold etc., as above.
Doesn’t more gold have to keep coming into the system to handle things like profit, interest, etc?
No. I make a profit by selling something for more gold than I paid for it.
I get interest by lending my gold to someone in exchange for getting a bit more of it in return eventually, as in above example.
Am I missing some important variable that would help explain this system?
You might be missing a good reading of What Has Govt Done to Our Money by Rothbard, where he discusses in a few pages how an economy runs under a gold standard.