If I insert a tube down the 47RE filler tube and pump out fluid, where will the fluid come from and how much am I likely to get? I do have a pump that will do it.

Doing this could help me in one of the following ways:

(1) If the fluid comes from the pan, I can suck out the fluid and it will make it easier to remove the pan without spilling fluid everywhere.

(2) If the fluid comes from elsewhere in the transmission, when I do a filter change and replace the fluid in the pan; I can then suck out what I can get out of the filler tube and replace it also, allowing me to change more fluid.

I'm asking this question because at this point I am hesitant to replace all the fluid using the procedure that requires disconnecting the fluid return line at the transmission.

 

Yes the fluid can be sucked from the filler tube. But, you will not get all of it. The converter is still full and will slowly drain/leak onto the floor when the pan is off. This makes it less messy to change the filter and do service work but that is as far as it will go.
i'd say to put in a drain plug the first time the pan is off.
most aftermarket converters have a drain plug also. this lets you drain 99%. don't get into the habit of the flushing process, as the filter still needs to be changed, as well as inspecting the band adjustments.