True. However, given the unbelievably huge amounts of water that evaporate and condense each day, the "human" contribution is so small it literally cannot be measured on a global scale.
It can effect climate in localized areas, for instance where irrigation become widespread in desert areas, but not the global climate.
Right. The minimum temperature will increase and the maximum temperature will decrease. Now if you have a data base of temperatures taken at 3 pm every day, the recorded temperature will be lower if the humidity is higher. Simply because more water vapor molecules increases the density of the atmosphere. A denser atmosphere requires more energy (sunlight) to reach a given temperature. It is probably easier to make sense of the situation by thinking in terms of air density. More dense air (higher humidity) can hold more energy, but will achieve lower peak temperatures for a given amount of solar input. So higher humidity will cause lower peak temperature readings at 3 pm every day and lower humidity will cause higher peak temperature readings. So the temperature delta (change) from day to day in this specific case is responding the exact opposite of energy delta (or change). Higher max temps, lower total energy. Lower max temps higher total energy.
You really are just playing with noise if all you work with are temperatures. Total energy or latent heat is the key. But hey, global warming is not about science. It is all about political science. Now imagine a planet if you will, that is heading into an ice age. Less energy means there should be less water vapor, since energy is required to get water to evaporate (turn into a gas). So a planet is cooling and the air is getting less dense. Temperatures if taken at 3 pm every day, might actually show an increase. Since basically the air is less dense and can warm higher given the same amount of solar input. So by the time you actually see big drops in maximum daily temperatures due to the onset of an ice age, it will probably be too late to prepare. So for any temperature change, must be biased or adjusted with any change in air density (humidity or heat content). Otherwise you might as well be throwing dice in Vegas.
Man that di-hydrogen monoxide is a killer.