To answer the OP's question on what would persuade Amazon to start selling EPUBs - several factors, one or more of which could prove persuasive to Amazon.
1.
Demise of the Agency model - Fixing the price of an ebook curtails the ability of book sellers to compete on price. Amazon cornered a big chunk of the pbook market by focusing on price. If they are unable to do the same with ebooks, they lose their competitive edge.
Reality Check: This is unlikely to happen. Amazon went with the proprietary format BEFORE the Agency model came into being. This means that they feel that price competitiveness alone does not afford them enough of a competitive edge.
2.
Buyer aversion - If buyers start voting against Amazon with their wallets, it may force them to review their business plans. There is a small but growing number of people who are discovering that you cannot read library books on Kindle. There is also a minority of technically minded people (including me) who see this growing into a monopoly if it already isn't one. Think IE 6 with no competition and a tonload of non-standard stuff or MS Office with the stranglehold on DOC, XLS and PPT formats or in very general terms the Apple business model.
Reality Check: Much of the market does not really care. They would be happy in the Amazon monoply till Amazon starts misusing it and then they would grumble but have no choice but to pay up. Then there are the Amazon zealots...
For library borrowing, my humble guess is that rather than adopt EPUB, Amazon would do one of 3 things :
a) Reach a deal with Overdrive/other library providers to provide MOBI/AZW formatted books
b) Buy Overdrive outright and convert it to all Amazon formats
c) Create a competing service to serve libraries
As for the minority of technophiles - do I even need to say that they do not matter?
3.
Technological Change - The two big ones that are looming are - the shift to color/large format/graphics intensive books and international growth. There is a nice post about the deficiencies in MOBI format earlier in the thread. How well Amazon adapts to these changes would determine if Amazon switches. If it sees that switching to EPUB is cheaper while not endangering its competitiveness, Amazon may switch.
Realiy Check: Amazon has a good technology workforce. It is unlikely that it would prefer to switch to EPUB than enhance AZW/MOBI.
4.
Business Landscape - Competitors reduce Amazon to a minority share of the market.
Reality Check: Currently, this is hilariously unthinkable. Amazon has a big first mover lead over competitors. Currently, not many of them have proved to be equals of Amazon, let alone better it. In the US, Sony has priced it's readers astronomically, Borders is teetering on the edge and B&N is not doing too great financially though much of that is B&M store related.
Gut Check: Amazon is not going to change its business plans unless something causes it to lose market share drastically and this does not seem like a probability at this time.