Is merely saying "yes" out of shyness to someone who describes something unlawful as normal, while knowing internally that it is unlawful and having no intention of deeming it permissible, considered an act of deeming the unlawful permissible? And does the lack of feeling remorse in this situation prevent repentance?

It is clear that the questioner is afflicted with the whispers of disbelief (kufr). We advise her to turn away from these whispers and not pay them any attention, and to ask Allah for well-being from them. There is no disbelief in what she mentioned. The disbelief-inducing permissibility (istihlal) is when a person believes that something forbidden—whose prohibition is known by necessity in the religion—is permissible.