The barrier is the volatility.  They still have to pay someone like coinbase to sell their coins immediately so as to minimize risk.  It's too much of an unknown, and the floor has dropped out of Bitcoins on many occasions, and they could lose a ton of money in just moments.  Retail is way too competitive to take on any risk.

None of the things you've suggested will minimize a retailer's risk, only a long track record of stability.  Overstock sells their bitcoins immediately, they get transferred to Coinbase and sold as fast as the transactions allow.  Their prices are linked to coinbase plus a healthy markup to pay for Coinbase's services and minimize risk.  For such a large retailer, though, it also doubles as advertisement

Many regulatory agencies around the world are concerned that digital currencies can be easily used for money laundering or the illegal purchase of weapons and narcotics, and have moved to control or prohibit their use in ordinary commerce.