My thought is that you build it the best you can with the constraints that you've been given. I think you've done your best to advise of potential pitfalls. I hate to say it, but if something does happen with it, you can redo it, but certainly not under a warranty status. You can then add the back or supports or whatever is required, I'm sure with a lot less resistance.
Remember, the customer is always right, until they aren't.
Rotating vane sensors work very well, but they do take away from the bin capacity. I picked up a used vibrating rod sensor and have done some prelim testing and it works consistently and takes very little in the way of height. I just have a 35 gal. bin, so can't really afford to lose any.