Ideally all three front speakers should be the same and at the same height. In LR systems this is not always possible but you should try to get as close to ideal as possible.
Do you have a showroom? If so then do whatever you want in your own home. If not then you should put in your home what you want to sell. I’m not suggesting that it will be an everyday thing but giving a client a demo can mean an upgrade of thousands or tens of thousands of dollars. One upsell can pay for your system.
Either go all Kef or all Triad depending on what works best for your business. If it was me I would go all Triad and have them custom build the center channel with the same drivers as the Left and Right. Think about the level of speakers your market will support and put in one level better.

Only if you know any better. When one argues like this it is hard to believe one knows any better.

I saw this demo at CES for a Dolby Atmos soundbar in a demo room. I have to say that it sounded impressive. All the overhead effects of Dolby Atmos were there.

I have attached a link to the demo I saw. It might fit the bill for what you are looking for. After hearing the demo I am probably going to put it in my house especially if it arrives at $1000 with an included wireless sub.

[Link: digitaltrends.com]

If the LR speakers image well and are relatively close to the TV, I don't miss the center. Unfortunately, the slim 'bar' type speakers, don't image well (in my opinion).