Based on everything that has been said here, if the customer doesn't want ceiling speakers... I guess they don't want ceiling speakers. Regardless that this will give them the best coverage in this case with the low ceiling.
Hopefully when this space is used as an overflow area they fill it up with chairs so people are seated instead of standing. With the low ceiling, that will help significantly with getting the audio to the back of the room.
I expect any of the mentioned slender and tall loudspeakers that have been mentioned should work well enough. I would mount them hard in the left and right corners, as high as possible. This allows you to use the side wall as a beneficial boundary since you will not have a late reflection off of it. Toe the loudspeakers in a bit so they cross fire to the opposite corners of the room. I suggest the corners also because I expect the doors are open when this space is being used as overflow. With the doors open, if you mount the loudspeakers in the middle, the coverage will be interfered with by the doors. When the loudspeakers are over in the corners, that will not be so much of a problem. I am attaching a cropped and resized version of your photo with red boxes drawn on it to represent where I would want to mount the loudspeakers if I were doing this project.
It will be whatever it is, and that is about all you can do considering the constraints that you are allowing them to place on you by dictating the type of loudspeakers (no ceiling speakers allowed), the budget ($3500), and the mounting (must be on the front wall).
Have fun and good luck!