As you understand, you have basically two choices:
1) Reach an agreement that everyone can live with, although perhaps no one will like.
2) Leave without reaching an agreement.
The process of attempting to reach an agreement (negotiating) should clearly be your first step. Are you familiar with the concept of BATNA?
In negotiation theory, the best alternative to a negotiated agreement
or BATNA is the course of action that will be taken by a party if the
current negotiations fail and an agreement cannot be reached. BATNA is
the key focus and the driving force behind a successful negotiator.
BATNA should not be confused with the reservation point or walkaway
point. A party should generally not accept a worse resolution than its
BATNA. Care should be taken, however, to ensure that deals are
accurately valued, taking into account all considerations, such as
relationship value, time value of money and the likelihood that the
other party will live up to their side of the bargain. These other
considerations are often difficult to value, since they are frequently
based on uncertain or qualitative considerations, rather than easily
measurable and quantifiable factors.
You should take some time to clearly understand what the best outcome is for both you and them if you are unable to reach an agreement. Presumably any agreement where both of you have an outcome superior to your respective BATNAs is superior to not reaching a deal.
If personalities are getting in the way of discussions, then perhaps an independent third party (mediator) can help all of you identify possible outcomes that are superior to no agreement.
You gave us your prospective on the situation, and we have not heard theirs. As they say , there are two sides to every story. I am not saying you are wrong; what I am suggesting is that they may see the "facts" differently than you do. If any of you proceed on your own without reaching an agreement first, and become successful, then the possibility of a lawsuit down the road is very real. In such cases the lawyers get very rich. Go watch the movie Social Network and consider the Winklevoss twins who got hundreds of millions.
Finally you state: "since I have never signed anything relinquishing my ownership of the code". Be careful, not all agreements need to be in writing. Oral agreements can not be ignored. Further a court might infer and agreement based upon the actions of the parties. For example in the previous question you stated:
This app is the thing that the other guys (who started the company
before me, but said I would be a co-founder) have been flashing around
to potential customers.
It may be that the potential customers inferred form these presentations that the company had the right to offer the application to them. Have you ever done anything to either support or dissuade this inference? If you have previously acquiesced to such presentations, then you may have given the company some rights in your software (like the right to present it to potential customers as theirs) already.
The real point is that if you proceed to use your software without first reaching an agreement things could become messy for all parties and the only big winners would be the lawyers. Things are seldom as clear cut as we would hope.