Don't worry about the trim tabs falling off, providing the screws are tight and secure all the load being applied through them is in shear. I don't know how many screws there are, or the diameter, but assuming they are stainless steel and around 3mm in diameter it would need tons of force to be applied for them to fail. Water pressure on the surface area in question will not be able to generate such a force. I could work it out but I'm on my third glass of wine and watching Grand Designs while I type, but I would be dissapointed if Mad Mike or Affishionado didn't do it for me.
Gordon H