You hired a "buyer's" agent that you found online, to help you buy what you expected to be a "second home", but in fact you found a home to replace your Primary residence. However, you don't feel that this "buyer's agent" would be a good "listing agent" to sell your current home. (although you didn't explain why).
In addition, no matter who you use, you want to pay 1.5% commission to your listing agent, and get a 1.5% credit from the house that you're buying.
1) Since you already saw the house with your "buyer's agent", even though you haven't signed an "exclusive buyer's agreement" there is some obligation (morally, ethically, and more) that this agent is entitled to some or all of the buyer's side commission, should you decide to purchase the home. Minimally, he's entitled to some type of showing fee, if not the full commission. Bringing yet another agent in, to help purchase the property would unduly complicate matters, so unless you feel that he's not a good negotiator and wouldn't be helpful in the purchase, I see no reason to muddy up the waters.
2) the commission format you describe is not common, the buyer's agent will be paid according to what the MLS listing has published, and the buyer is not entitled to any credit therefrom. Whatever your agreement with your listing agent is certainly negotiable, and there are many reduced commission formats available out there, from ZipRealty, Redfin, BuyOwner, but other than a discount brokerage, who won't do dramatically more than stick a sign in the ground and throw you on the MLS, I don't know of any full service agencies who will accept a 3% list/side agreement and rebate 1.5% back to the seller.
It really depends on what paperwork you have or have not signed. You may sign an exclusive buyer representation with an agent, but that shouldn't obligate you to list your house for sale with the same agent. If you have signed an exclusive agency agreement, then you might be stuck.
First, depending on the laws in your state, your current buyer's agent could be entitled to the commission from the sale regardless of whether or not you have signed a buyer's agency agreement.
I'm glad you are thinking about the moral side of switching agents. I think you realize it would stink to invest time and work into a transaction you would not be paid for. Have you expressed your concerns to your agent and given him a chance to address them? Or have you gone on the word of others? I went to doctor once because she was glowingly recommended by a friend. My experience with that doctor was more than terrible, but she was a good fit for my friend. Word of mouth works both ways.
My advice? Meet with your agent, honestly tell him your concerns, and see if he can address them. If after that you still don't feel comfortable, then break the business relationship personally, honestly and quickly. Don't leave him to hang or let him find out when the home sells. Not every agent is for everybody.
About your second question: It isn't common, and don't go there. Consider that agents take on personal expense to market your home in the form of advertising, which is not cheap and comes out of their pockets directly. Instead, feel free to shop around for the best deal -- just let the offices and agents you talk to know that's what you're doing. And consider more than commission rate in choosing someone to market your home -- ask the agents about their basic marketing plans and what they would do to get your home noticed. Different agents in the same office can offer different levels of service at the same price. Some will include extras their competitors won't. It's not all about money.
Also, think about the buyer's agent point of view -- they don't want to sell a house they're going to make a pittance on. They may tell their buyers about your house, but it may not be the one they talk about most favorably. They may show it, but it won't be first in the rankings. A fair commission split between the buyer's and seller's agents is in the favor of everyone.
And would you do your job for half the pay? Real estate agency is a regulated trade, not a racket. A skilled negotiator can SAVE her clients a lot of money. And she invests time and money into state-required and additional education to do it to the best of her ability and protect your interests. You may find that amount you think is enormous was well worth it when you are relaxing in your next home in a deal you could live with.
Merry Christmas, and best wishes for your homebuying experience!