If you overfill the battery, will this cause more gasses to escape?

Any excess gasses are likely less of an issue here, these would just vent as usual. What is more important though is that if you overfill the battery too much your effectivly weakening the acid solution which is how the charge is maintained and transported within the cell.

Also an overfilled battery is more likely to leak as it is charged, so the sulphuric acid content is more likely to cause issues as it is very corrosive. It will eat any paint etc it comes into contact with and depending on the amount and its acidity, can easily start to eat into the metal bodywork over time. To prevent this flush any leaking acid away as soon as possible with lots of clean water.

I can't recommend you try draining any excess battery fluid as sulphuric acid can cause skin burns etc and will certainly eat any clothing etc it comes into contact with. A battery specialist or good garage however will likely be able help, and drain a little excess liquid from the cell for you, just to prevent any future bubbling over of water/acid.

They can also test the specific gravity of the acid in the cell with a hydrometer which will help indicate its condition and whether the excess overfilling has inadvertently weakened the cells performance.

The answer is yes and the results are messy and potentially toxic and corrosive. The only time you add water to a lead acid battery is when it is fully charged. The reason for this is when a battery is fully charged the plates are thicker and the there is less space between them. The electrolyte level is at its highest.

When discharged the reverse is true. The electrolyte level is at its lowest. Adding water at this point would cause the excess water to rise and exit the batteries vent.

I have noticed when filling batteries with water after charging that the middle cells seem to be the driest...No idea why though. Using a hydrometer (looks like a turkey baster ) add water just until the level is just about the plates. A space at the top of the battery allows the hydrogen gas to escape.

Too much water causes more gases when charging , such that you upset the specific gravity of the acid in that cell causing it to produce a lower voltage causing the gas.It will evaporate eventually at the expense of battery service life being lowered.