I want to follow proper agent-writer etiquette, and I've rummaged through many a Google search trying to find the answer to this question:
Let's say you are lucky enough to receive more than one offer of rep from an agent. Happiness and joy! And you click with both Agent A and Agent B, but ultimately choose Agent B.
Two years go by, and the manuscript that got you the offers doesn't get picked up, but it also doesn't get shopped around much either. Only three editors see it before Agent B gives up on your manuscript, and since you weren't an easy sale, Agent B seemingly gives up on you. You and Agent B part ways.
Is there any hope in contacting Agent A and hoping for rep, or is said manuscript (and said agent) a lost cause because of the rejection?
Oh, how loud the lamentations when you realize you should have picked meeeee!
This happens more than you might imagine. Not always for the reasons you list, but I hear about this kind of email from writers Who Signed Elsewhere more than rarely.
My advice to the agents who are being requeried is to always have a conversation with the writer. Figure out what went wrong with Agent A. Figure out what the prospective client is now looking for with Agent Two. Take a gander at the sub list and see if there's money to be made.
Each case is so individual there's no real standard answer.
The answer to your question is however: yes of course you can reestablish communication. The worst thing that will happen is
It is not rude to do this.
What querying writers should remember whilst querying is that you may end up circling back to agents you said no to. It is of the UTMOST importance that the agent remember you as positive and professional.
Things that say Positive and Professional:
1. You gave all the agents considering your work a window of response time when you received an offer.
2. You notified them politely of your decision.
3. You did not immediately unfollow all the agents you were stalking on Twitter. (This is something other agents seem to care about. I don't. Follow/unfollow/mute/magnify it's all the same to moi.)
4. You did not add those agents to any sort of mailing list.
5. You were tactful in announcing your decision on Twitter: "I've signed with the BEST AGENT EVER" is seen with less glee than you might think by all of us you turned down.
6. You did not whine/moan/carry on in any public way about the travails of your experience with Agent A. In fact, no one reading your blog or tweet stream or Facebook posts would have a clue anything was other than hunky dory.
As to the viability of your manuscript that is too specialized a question to answer here. It depends on too many variables. It's probably best to assume it's not viable and you should query with a new project.