A few questions regarding the transit of planets
When we talk about a transit, we are normally talking about one that is visible from Earth, or at least from some planet in the solar system.

But if you are willing to go to a point is space, it is always the case that every planet is transiting the sun from somewhere. So the Earth is right now transiting the  sun from any position that is directly opposite the sun and more than about 1600000km from Earth (otherwise the sun won't be transited, it will be eclipsed.

The transits of all the planets can be seen from exterior planets, though the transits of (say) Saturn from Neptune are exceedingly rare

If you are in the orbital plane of the planet, you will eventually see a transit.
But you may be waiting a long time. The semi major axis doesn't have any effect.

If the planet in a planetary system are in the same plane, then you may see more than one transit. But you are less likely to see the outer planets as they are further from the star.