I am not sure about the full shop in shop solution. I still think people want to feel they are walking into somewhere a bit special, and agree with the points about staff training etc.
I would have the store still primarily B&O, but dedicate some space to other solutions and make it clear that the store can help customers with a carefully selected range of other products. I don't want or need B&O TVs for my kids rooms but I want to deal with the same person who looks after the rest of my house. Most dealers can supply other brand panels if asked, and they now need to supply other brand relievers to work a BeoLab 14 so let them show it in the stores.
I have some sonance invisible speakers at home supplied by a B&O dealer. He doesn't have them on display in the showroom though as they are not part of the B&O shop fit.
I travel a lot with work, and probably visit dealers in towns I visit when far more than my own dealer who only gets a phone call when I need something as there is no chance of there being anything in the store I haven't seen before. General response when travelling and asking dealers is that they all supply other brand kit when required anyway, but aren't allowed to have it in store. Seems stupid to me, even if B&O organised a group buy through a reseller for them so they could limit the models and keep them high(ish) end and give the dealers a decent margin with some group buying power.