An agent must present all bona fide offers for consideration to the seller - and until the time the the contract is in force, here in NJ that is after the completion of attorney review, either party can shift directions without recourse. So the answer to your question is that "yes" a seller can accept another offer during that review period. It is the seller that makes the decision on whether to accept an offer, not the agent. You are questioning the motivation of the agent, and while it would be rewarding to her/him if she collected the entire commission, that should not be the motivation. The agent presents the offers - and incidentally your agent can ask to present in person (I always ask to do so), but the seller makes the decision on what offer is the strongest.
You can request that your offer remain as back-up - it is a long road from acceptance to close, and things can change along the way - as you have found out. The contract likely has an inspection and mortgage contingency (unless all cash) and these are two important performance hurdles when transactions can hit bumps.
I always advice my buyers to urge their attorney to get them in and out of attorney review quickly so they cannot be bumped out.
Good luck to you!
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