Tim asks "1.  How much of a problem is it if the syrup gets a bit moldy?

2.  How much does vinegar extend the life of 1:1 and 2:1 sugar syrup,
and does it have any undesirable effects on the bees?"

I have no idea if the mold or fungus is harmful.  I think it affects the
taste and it will get to the point when the bees will not take it.

I use 1/4 cup to 5 gallons of syrup, which is a tiny amount.  I have never
observed any adverse affect on the bees, and it doesn't seem to inhibit
their taking it.  (I can't taste the vinegar at this dilution.)  It is hard
to say how much it extends the life, but after two weeks I don't see any
mold in what little syrup is remaining.  I don't think I have had syrup on
for longer than 2 weeks...in other words, I feed amounts that are consumed
in two weeks or less.

Hope this helps,
Lloyd