You can work with more than one Realtor at a time, but it may be better to talk to a few first, then choose the one you like. If it doesn't work out as seems to be the case now, and you haven't signed a buyer/broker agreement, you're OK, and you're free to interview more.
It's really important to have an agent who can plainly explain the process to you, and to let you know what issues could arise, so you're not surprised down the road. I also feel that it's important for an agent to be able to understand the loan process, enough to avoid getting into escrows that won't close. We can't control everything that happens, but we can do the research that will help avoid unnecessary escrow cancellations and costs.
As far as showings, if it takes an agent a week to be able to show homes, they're working with too many clients. In the first time buyer market, it's sometimes essential to get offers in quickly. The good ones go fast when they're not short sales, as you already experienced.
The wait for return calls and emails is inexcusable. Really. You're not being too needy, you're being taken for granted because of the referral. It's time for a new agent. A bonus would be for your agent to answer your calls as often as possible!
Don't just work with the listing agent because it's easier, it's not a good idea. There's no reason to, the only one benefiting from it is the agent, who's getting double the commission. How can the listing agent get the most money for their seller at the same time they're supposed to be saving YOU money? It doesn't cost you anything for your own agent, and you really want your own negotiator, a tough one!
Call anytime, I'm happy to start showing you homes, and getting some offers submitted.