Who has said that she only premeditated the murder with a gun? The knife used on TA did not match any of the knives in his apartment and no knives were missing from the apartment. This leads to the conclusion that she also brought the knife into the home with her.
The trajectory could be from either prone, supine, side lying, or sitting. All we can definitely say about it is that she was not floating the air over top of him. If he is lying on the floor with his face turned to the right, the trajectory is not real hard to see. If he is sitting in a reclined position, the trajectory is just as possible. The trajectory is roughly 20 degrees to the right from true vertical. That would not be an easy angle for her if he was sitting upright in the shower while still maintaining the minimum 2ft horizontal distance from him. If he was slumped down (butt further from the wall) in the shower, it would become easier. With him on the ground, he would either have to be on his left side or have his head rotated right. This would expose the area from midline of the brow to the lateral ridge of the Frontal Bone (the area where the bullet entered the skull). With him on the ground, she has the freedom of movement to be at any angle when the shot was fired. Stepping back to 3-4ft makes the angle rather easy to see. The trajectory can really be reproduced with him in just about any position. Him sitting upright would be somewhat more difficult.
I cannot give you her thought process anymore than you can give me her thought process. I don't think like her. Plenty of people have chosen a knife for a pre-planned murder. She has already stated the he refused the offer to be tied up, so that was not a real option. I can come up with multiple options that are better than killing him in his bathroom, but I didn't pick the place or the method, so I can't tell you why she did it. But I can tell that she did use both a gun and a knife on him, and she killed him with a knife.
So far, I've only seen the gun firsters (yeah really, firsters is a real word. it just looks and sounds so weird) proposing the stabbing started in the shower. I don't believe I have read one proposed knife first opinion where she started stabbing him in the shower. But, I do agree with you that his vital organs were better protected while sitting in the shower. Now, his hairline might have been exposed to a cut while sitting in the shower. I don't think anything happened in the shower be it with gun or knife.