"The figure shown in the trifecta pool at the time a race jumps should be fairly close to the total available for distribution - is that correct?"

I actually think that is the total betting pool before the tote's cut. With trifectas approximately 20% is taken out.
You should be able to confirm one way or another about this with your TAB. I am sure everyone here would be like to know for certain whether that tote pool they see is all returned in dividends.