There is more than gravity at work. There is the energy absorbed by each body in the solar system and the solar field.

The sun's magnetic field is simultaneously generating an electric current due to magnetic hysteresis. This arrives to us in one form as light - electromagnetic energy - and in other forms of electromagnetic energy that are not visible.

Maxwell's correction to Ampere's Law points out that each change in polarization state is the equivalent of a current. The sun constantly oscillates in its powerful magnetic fields. The objects in the solar field absorb and distribute a quantity of this current amongst themselves. If one of the planets were to disappear, or even to significantly adjust their orbit, the dispersion of polarization density would differ based on their location in the field.

So in simpler words if Mercury disappeared the Earth would pick up a charge - we would 'inherit' more EM energy. This could increase our orbital revolution and this would alter our climate - possibly drastically. It would alter the rhythms of the animals and plants. If Mars was moved closer to the Earth, the Earth would likely lose some of the field polarization density it absorbs and our orbital revolution would likely slow down - again causing calamity.

Our sun is a giant field generator. Its hysteresis generates enormous electrical current that is dispersed as light through the field. Bodies in our solar system interface with this energy in multiple manners. The linear light that hits a body that falls in its path is one type, but another type of electromagnetic energy is drawn unto each body based on its new position in the field.

The solar field (aka IMF) is, naturally, vortex shaped because the sun revolves every 27 days. It's energy is dispersed as solar wind in this fashion and as each planet revolves around the sun it slices through this field and its various gauss levels. Depending on the polarity of the solar wind as it hits the Earth's magnetosphere at the magnetopause, our magnetosphere shielding is cancelled out somewhat if the solar wind has a southern polarity. When this happens, because our magnetosphere points north towards the sun, huge amounts of solar wind break into our atmosphere and cause the breakdown in the ozone layer, northern and southern lights, etc. It's not all bad because this constant cycling feeds energy into our magnetosphere and strengthens it with more energized particles.

Cycles are everything. Mess up with the existing orbits and cycles and one will find out just how interconnected we are to other worlds in this field we share. This is why the global warming folks are all wet. Any of a thousand factors that strongly influence our weather are almost completely unknown to them.

Creating an artifical electromagnetosphere, as I mention in my post, would also alter the balance of things and would absorb more of the solar IMF current robbing some measure from the Earth and the other planets.

Here's an easy analogy. You have a 100 watt line. When 5 bulbs are placed on it each can use 20 watts. When 6 are on it that figure drops to 16.7 watts. When only 4 are put on it they can use 25 watts. In this scenario any manipulations of planets or their magnetospheres in the IMF will ALWAYS influence ALL the other planets to a small or large degree. These changes could manifest in all sorts of ways, temperature, orbital speed, magnetosphere strength, etc.

Moving Mars closer could mean the end of life on Earth by weakening our magnetosphere.

So as you can see, gravity is only one of many factors that need to be taken into account when trying to play god with the planets. ;-)