Tri shoes are just like road shoes in terms of their bottoms - so - they're just as hard to walk in. The real difference is that they close with a large velcro flap and typically can be opened up easily to slip in during transition.

You can always wear mountain biking shoes. You'd be much happier for commuting and there's no real problem with wearing them for road/tri cycling. I wear a mountain shoes that fit me really well and have been more comfortable than any road shoes I've found. Maybe the snobs will look at you funny but there's nothing wrong with it. (and if you're doing triathlons, you'll see a huge range of gear...there's much more variability than in the road scene)