A simple instantaneous addition of an Earth-mass planet with an orbit located somewhere in the middle between the orbits of Earth and Mars, moving in the plane of the ecliptic in the same direction with the first cosmic (for its distance) speed will not change anything. It is easy to verify this by conducting a simulation in the Universe Sandbox (https://universesandbox.com /) or using other means of simulating celestial mechanics and correctly scoring the characteristics of the bodies.
The whole effect will be limited to the appearance of another wave component of motion in the population of the solar system, this system is stable in the future for billions of years. Is it possible to add this planet «naturally», that is, to capture a successful series of gravitational captures flying through the solar system (primarily from Jupiter and the Sun), and even without a critical increase in the eccentricities of the orbits of the existing planets?
Apparently not: to «drag» the planet to the right distance (and even to the right aphelion point) it is still possible, whereas sharply reducing the eccentricity of the orbit at such large distances seems to be a completely impossible requirement.