Hello again Bill,
So you've discovered how much output the F2B has. It's quite a bit of gain! I think it was designed to be the last in the chain directly driving the power amp. But again that shouldn't stop you from experimenting.
Since the F2B can be run truly in stereo, you can also experiment with connecting the output of channel A to the input of channel B. Running the A channel volume high and the B channel very low can introduce some serious crunch - if that's what you're looking for. But again, the output of that B channel will be a very high level and may not play nice with your other pedals.
That JHS piece does look like a simple line level attenuator which might work as you describe. I'm going to guess the reason they say it shouldn't be used with other pedals in the same loop might just be because it effects the impedance, so the pedals may not see the "load" they're expecting and results could vary? Not sure about that, just guessing.
I'd also like to suggest you try the chain you describe above but move the F2B to the end, just before your combo "return" input. That would certainly make your pedals happier. And again, you could chain the F2B's channel A & B together at that spot for extra crunch. You may find you don't even need your "amp sim" pedal that way.
There is a simple way to step down a line level that involves a couple resistors in an "L-pad". Not hard to make but I'm not sure what the values should be. That might be what that JHS pedal is doing anyway.
Keep experimenting!
Jimmy J