They sure can, but they never should be! Stick your head up thru your attic access hole or better yet, climb into your attic (being carefull to step only on your ceiling rafters) and take a look. If you can't see because of the roof slope, just eliminate any light and you should see light shining up thru the vents from the outside. If you don't see any then it's likely they're blocked. There should always be good air cirulation thru your attic!
Soffit vents can be painted as long as they are appropriately primed based on their existing finish.
With either gable vents, soffit vents or ridge vents. The more the better
No, but they all should have.
Most soffit material is now made with a continuous 3" wide vent.
The vents allow the loft space to breath. By blocking it can cause damp air to accumilate and thereafter you have condensation damp and mould.
The reason baffles are used is so the blow in insulation does not cut off the air flow coming in from the soffit vents. If you are satisfied that your batts provide enough flow at the eves then call it good. This assumes you have ventilation at the ridge, otherwise it will not be very useful.
WHICH VENT If you're talking plumbing roof vents or soffit or ridge attic air vents; absolutely not!! I can't thing of a single vent that would be opened and closed at will.
It is recommended that you have both intake vents and exhaust vents. Most do it yourself sites recommend soffit vents for the intakes. Exhaust vents vary in both type and cost. For smaller houses the most recommended type is a ridge vent system. The number you will need depends on the square footage of your attic.
Soffit
Soffit vents are located n the soffits, which are the horizontal surface between the top of the wall and the edge of the roofline along the bottom line of the roof. Usually the same roof edge line where you may have gutters installed. These vents allow air to flow into your attic to provide ventilation which may then flow out the ridge vent or gable end vents. You can see an image here: http://www.cornerhardware.com/howto/images/ht076_2.jpg
the roof is required to have venting (size) based upon the square footage of the ceiling area
Either you have a blockage present or your vents on your sewer lines have become blocked or you need to install more vents.