The brisket won't fit.

[Rich Dahl]

And it will shrink a lot more when you take it out and eat it. I have an 18.5 WSM also and the first time I smoked a 16 pounder I was sure that there was no way that puppy was going to fit.
I did the same as you, I asked for help from the more experienced brisket folks and basically got the same answers you got. That's what makes this forum so great, friendly people with a lot of knowledge and their willing to share it.
Oh and one thing not mentioned, even though you bent the hell out of that brisket it will still be delicious.

[Mike in Roseville]

I have never met a brisket that didn't fit on an 18WSM.

Glad you have "seen the light." Now enjoy the fact you can finish that on one ring of fuel.

[Gaaron G]

Here are a few photos of my cook. You can see in the first photos why I was so concerned. In the end it turned out delicious. It was the best brisket I have ever had. The picture with the umbrella shows what I had to do to keep the temps down with the early morning sun hitting the WSM. 11 hours in total on the BBQ. (wrapped in foil at 8 1/2 hours). 3 3/4 hours in the Ice chest. Silky like butter.

https://drive.google.com/drive/folde...d-?usp=sharing