I would encourage you not to do that job. It is likely the customer won't like it and blame you. The fact the width of the room is larger than it is long is a big problem as well as all the bass leakage. That sounds like a room that is totally unsuited to multichannel audio. It sounds like a living space area, and in that case is only suitable for two or three front speakers and a sub. In a space like that in wall works best and has the best WAF.
Built on the orders of my wife, and she loves it. Two way left and right, MTM, and three way center and in wall TL sub. That is the way to handle a space like you are asking about.
Yeah, I’ve had a similar situation with a client before. Room layout wasn’t ideal, and they insisted on sitting behind the surrounds, which definitely throws off the soundstage. In setups like that, it’s all about compromise and clear communication. I explained that the rear imaging would be weak or even distracting.
What worked for me was recommending a strong front stage and ceiling Atmos speakers to compensate. Also suggested repositioning furniture a bit forward if possible even a couple feet can help.
Interestingly, I’ve worked with another client on a full home setup where the layout was open-plan too, and we had to plan around awkward dimensions and traffic flow. For some context, they also needed chauffeur perth airport services regularly, so the home was always in use but had to stay clean and accessible.
Back to your case if the client’s willing to rethink the seating a bit or even bring the surrounds forward with angle adjustments, you might still get decent results. Otherwise, set the expectation that the rear channel experience won’t be ideal.