A clothes dryer vent should not be very humid. If it is, it may be clogged. Clean the dryer vent once a year to prevent this among other problems such as loss of efficiency and of course FIRE.
Safe is a relative word. Crossing the street can be safe, depending on whether or not a car is coming. Likewise, venting a dryer indoors will increase moisture levels as well as introduce dust/lint...
Yes.
It is the moisture from the clothes and what little lint the filter doesn't pick up that you are venting. If you don't vent it outside the moisture could lead to fungus buildup and other not...