Keep in mind that on mountain bike shoes they have tread on each side of the cleat so you can walk around:


No side tread on road shoes but they have a larger cleat that spans the width of the shoe:



If you attach a mtb cleat to a regular shoe, I would think it would be worse to walk around in off the bike than either kind of bike shoe.

The benefits of clipless are not that high, if you do some research. They make sense for racers where it's primarily keeping their foot attached to the pedal when sprinting wildly and they may or may not be able to slightly faster. If clipless works well for you without problems great, but if you are working real hard to try to make clipless work it's almost certainly not worth the effort. Clipless does not provide benefits in speed or efficiency to your average rider.

If you race, or almost-race, I'd say you'd be better off googling others with extra extra wide feet. I think some people sell shoes that are extra extra wide, where you can return them if you haven't put a cleat on them (so you can order them and try them on and return them if they aren't wide enough).