Even if the police informant elicited the information in the jail cell when he was not uniformed so as to avail the defendant of the knowledge that he was talking to a cop, it is still admissible. This is the case even if the defendant requested council - the idea here is, confessions cannot be forced when a defendant believes he is under the duress of police custody; if he does not believe he is being forced to talk or threatened to talk, there can be no duress, so the evidence is admissible. And the officer can testify to what the defendant said, but it has to be in the form of exceptions to hearsay evidence. As he would be testifying to matters for the truth of the matter asserted, they have to meet hearsay exceptions - the most important one here would be admissions of a party opponent.

If the defendant were to admit something to the officers in the jail cell, then it can be considered nonhearsay - not an exception in this case - and be admitted and survive upon appeal to the supreme court.

If the evidence does not qualify for a hearsay exception, then it can still be used, but only to impeach or for other purposes and not for the truth of the matter asserted at trial.

4) In this case, the illegal questioning of the defendant would preclude usage of the evidence. The cops questioned the defendant when he could reasonably be considered under arrest - i.e., not free...

As a result, he was to have been read his Miranda rights, and as the police officers failed to do so, he cannot be convicted based on this evidence alone - it fails the harmful error test.
Also, without a warrant, the cops may not have had the right to search the house and question the man anyway. Did the woman who let them in have the scope or the power to let them into his quarters? If he lived in a boardinghouse and that was his room, chances are she exceed her scope in allowing them to enter.

As a result, the admissions of the weapon and where the man had been that night could only have arisen after his Miranda rights had been read, and after his attorney was present if he so chose.

In this case, his rights were violated and the result is his conviction - harmful error since the police may not have been able to obtain that information in any other matter.

Under both fruits of the poisonous tree and Miranda, the Supreme Court must allow defendant to exclude this evidence.

Basically, the idea behind harmful evidence questions is a bit more anti-defendant that can be presupposed - the police are generally given a lot of leeway in errors in obtaining the evidence.

Bibliography

Barbri Criminal Procedure Review. (2005). New York: Barbri.

PMBR Criminal Procedure Review. (2005). New York: PMBR.