Google-sheets – IF Cell = True, then retrieve (separate) Cell Value

=VLOOKUP (a1,range,2) 

The problem with that is if the value isn't listed, which most of the time it isn't, it will fail. I want to return the closest value, which VLOOKUP doesn't appear to do

First, sort range in ascending order. You could do in place or use SORT

Try

=VLOOKUP(a1,SORT(range,1,TRUE),2)

If I understand your questions correctly, you want to be able to change the value in F2 but keep a history of that value in the 'C` column. All this, while all other cells update automatically by dragging down the cells.
In that case, I don't know a possible way.

What you could do is to not get the C column value from the "Current Gold Price" value in the F2 cell, but perhaps have it pulled directly from the source. It depends on how your source is. If that is a column that gets updated based on the date then use it as a reference just as you are doing with F2.