DP
I've had an used my WSM (same one) for more than 10 years and have never had the Weber water pan fall into the coals. It is true that the fit doesn't seem that secure because of the lip not overlapping the brackets. However, if you carefully position it on the bracket, pour your water (if you're using it) slowly into the center of the pan and don't try to move the center section during a cook, I don't believe you'll have a problem. I bought a larger Brinkman charcoal pan to use on longer cooks because of the volume. I now use sand, primarily, and use the Weber pan.
Also, put the pan in the middle section, install the middle section and then pour in the water.
The Weber pan is made of high quality, heavy material and I don't see it as a problem if you pay attention to what you're doing.
Just my thoughts. You have a great cooker - enjoy it !
Paul