You can hire an attorney to represent you as a buyer. Often, that attorney will handle the closing as well, which is a generally accepted practice because at the point of closing all of the key terms of the transaction have been negotiated (with the attorney representing the buyer) and at closing the lender and buyer's interests are seen as aligned. Mainly, both parties want to make sure the buyer gets clean title.
There are too many conflicts of interest, in using the same agent.
I would not recommend the dual agency relationship. I never understood how a Realtor can negotiate on the best behalf of two opposing parties. I stay away from those kind of scenarios. I'd rather have my clients represented by another Realtor if they wanted to buy one of my listings, than to have them have doubts about having received full representation by me. I believe most clients appreciate me looking out for their best interest and will come back to me for their future real estate needs.
Option two is the agent will remain as sellers agent and will become facilitator to you. Meaning He/she is still working as a full service agent for the seller, but is only facilitator to you, meaning you lose the "negotiator" part of the service, but the seller still get their.
It's also possible that the agent will be willing to work with another agent in their office for negotiation part, ie, your agent would represent the seller, and some other agent would represent you in the negotiations, but your agent would handle all the paper work etc.
Benefits? Depends on a situation, if you are offering near or full asking price, and have no contingencies (other than the usual mortagage and inspection) then by working with the same agent, you may get an easier/faster transaction, since your agent wwouldhandle everything and won't need to wait for listing agent to get back to them etc.
But that's about it. Seller might have it in their contract that if the listing agent gets both sides of a commission than the total comission is lower (ie. 6% normal, 5% if agent gets both sides) but that has no impact on you since you still pay 100% of whatever offer you make.
I would suggest you talk to you agent, see what they would do if you were to buy their listing. Although, if you plan to lowball or negotiate a lot, I'd suggest working with a different agent, and your current agent could get a referral.