I have cooked quite a few briskets... I have never marinated or injected one though. Here are my comments on your comments:
Wrapping the weber rack in foil is ok as a diffuser but you need to make sure there is an inch or so around the diameter that is open. You don't want to put too much of a damper between the meat and the fire. I believe that the weber rack completely wrapped might be a little too large of a diffuser. I highly recommend the $4 pizza pan from walmart as a diffuser. It's cheap and it's a perfect size. Set an empty drip pan on top of the diffuser just to keep all that grease out of the firebox...
You don't need the water pan in the kamado. The kamado keeps your meat moist on its own. It's part of what makes a kamado a kamado
I typically trim off about 80-90% of the fat cap on my briskets. Fat cap up or down seems to make no difference in my finished briskets. I have tried them both ways.
I have never mopped a brisket during a cook.
185-190 is a little low. I never pull mine before 195 and they sometimes go to 200 or even 205 before they get probe tender. Each cut of meat is different.
I get the feeling from your listed technique that you are worried about the brisket drying out. Don't worry! It's not gonna happen unless you over cook it.
225 is also OK and you can expect that brisket to take 13-14 hours to cook at that temperature. If your pit temp gets up over that, don't worry about it... just let it cook. I cook anywhere between 250 and 300 and the results are always outstanding. I have also cooked at 225 but I just don't do it that much anymore unless I need to extend my cook for some reason.
Good luck with it!
Let us know how it turns out