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.
I have the old Oneida mechanical sensor that spins and it works great once it senses the bin is full it sends a signal to a flashing light inn the shop. Not sure if they are still available but definitely works well. The only down side is it hangs down about 5" so reduces capacity.
That makes sense, but I'm a hobbyist, and I generally do a little work one day, and later do a little work on something else, and so on. So there's no real good end-point to check the dust collector. And there are other people who use my shop. So, for me an indicator makes sense. "When leaving the shop, check the indicator" is much more convenient than, "when leaving the shop, dis-assemble the Frankenstein dust collector and check the bin."
As you say, personal preference.