I believe Nibiru is a dark planet, with an orbital inclination almost perpendicular to the eccliptic plane, and with a very high eccentricity. These 3 combinations make it very difficult to detect Nibiru, whether directly or indirectly.

To help others imagine what a perpendicular orbital inclination with a very high eccentricity is like, try drawing the sun and the planets (with their orbits) on a paper. Now point your pen towards the center of the Sun and perpendicular to the paper, and start moving your pen closer and closer to the center of the Sun. This is an extreme case scenario wherein the eccentricity is set to infinity (straight line orbit) and the orbital inclination is perpendicular to the eccliptic plane.

Now notice that the tip of your pen never moves accross your paper nor does it cross the orbits of any of the planets. It simply moves closer and closer to the Sun.

Take note also that the tip of your pen (which corresponds to Nibiru) is always closest to the Sun, then to Mercury, then to Venus, then to Earth, etc. in this proper order.

Because of this, the gravitational acceleration (perturbations) caused by Nibiru will be strongest with the Sun, then Mercury, then Venus, etc. again in this proper order.

What will be the resulting perturbations on the planets if Nibiru's orbit is like this?

If this is the case, then Nibiru will cause the Sun to move perpendicularly with respect to the eccliptic plane, while the Sun drags the planets (and therefore the eccliptic plane) along with it.

In other words, such perturbations of the planets and the Sun relative to each other will be very difficult to detect. We may in fact wrongly conclude that these perturbations are part of the Sun's and the planet's natural movement along the Milky Way galaxy.

So how will these perturbations appear to us? It will appear like the eccliptic (along with the Sun and the planets) has moved perpendicularly with respect to the background stars. But since the background stars are very far away, this perpendicular movement of the eccliptic with respect to the background stars will be very difficult to detect.

Now since Nibiru is not perfectly perpendicular to the eccliptic (which as I said earlier is an extreme case scenario), we are in fact seeing small perturbations on the planets orbital eccentricities, but these perturbations will be much smaller than one will expect if Nibiru's orbital inclination with respect to the eccliptic is zero.

My point is this, the higher the orbital inclination of Nibiru with respect to the eccliptic and the more eccentric its orbit is, the less detectable will be its perturbations on the planets.

Also, the perturbations caused by Nibiru, will be more detectable on the Moon, than the outer planets like Saturn, Uranus, etc.