If a client admits guilt to his attorney, the attorney cannot put him on the stand if the attorney knows the client intends to lie. The attorney also can not plead the client innocent and present an alternative theory of the case. The attorney can plead innocent because of mental defect, diminished capacity, etc. and attempt to win the case that way.
CT: But the lawyer can put in a plea of not guilty and make the govt prove its case.
Also, not sure what you mean they can’t present an alternative theory. The defense can still present true evidence and argue it indicates something.