- Q1. The statement is not true. In 3D, if you impose the restrictions $$\|\dot{x}\| = 1 \,\,\, \text{ and } \,\,\, \|\ddot{x}\| = \kappa_0$$ then there are infinitely many curves with this property that are very far from being helixes or circles and having differing geometric properties.
This follows for example from the canonical Frenet-Serret frame of a smooth curve and the corresponding Frenet-Serret differential equations. Here is the link to an old post of mine about the Frenet-Serret frame and the equations. You can also check out Wikipedia on the topic. The Frenet-Serret frame and the corresponding equations are the motor that drives the so called Fundamental Theorem of the Differential Geometry of 3D Smooth Curves. According to this theorem, a 3D smooth curve is determined uniquely, up to translation and rotation in 3D space, by two functions: the curvature, $\kappa(t)$ and the torsion $\tau(t)$. What you are asking in your question is about the possible geometries of 3D smooth curves under two constraints.
(1) The condition $\|\dot{x}\| = 1$ is equivalent to the fact that the curve $x = x(t)$ is arc-length parametrized, which can always be arranged, so this is not a constraint on the curve's geometry. It is just a very useful coordinate choice of parametrization of the curve. In fact, when studying the geometry of curves, arc-length parametrization is most of the time assumed, in order to simplify calculations and expressions.
(2) The second condition $\|\ddot{x}\| = \kappa_0$ is a true geometric constraint and is equivalent to the fact that the curvature is set to a constant $\kappa(t) = \kappa_0$ (here we are assuming that the curve is arc-length parametrized, so condition (1) is assumed).
However, even after imposing these two restrictions, you are left with the freedom to select any (let's say smooth) function $\tau(t)$ as torsion. If you select the torsion to be zero $\tau(t) = 0$, then you obtain a circle. If you set the torsion to a non-zero constant, you get a helix. But if you choose something much more involved, then... well I think you get my point.
Let me put it this way, under the two constraints of arch-length parametrization and constant curvature, the space of all curves with these two properties is parametrized by the space of, say smooth functions $\tau(t)$ which is an infinite dimensional space. For each choice of torsion function, you get a curve with a unique geometry, up to rotation and translation.
So If things do not work in 3D, then imagine the complexity in higher dimensions.