[Delphinus]

Yes to basically all your questions.

Sometimes cover glass is necessary, ie., if you have fish that are known to be prone to jumping or attempting escape from their glass prison. However even then there are no guarantees. Some people prefer glass covers to keep evaporation in check.

But not only does a clean piece of glass reduce some light, anything that gets on it will too, not the least of which is salt creep. If you want to use glass you'll need to clean it on a regular basis.

I prefer open-top myself, but it's better to find your own comfort level.

FWIW, I wouldn't trust the glass to hold up a light, I think it's better that a light has some structural support other than the glass over the tank - if that glass gets bumped or something, you don't want that light to fall into the tank - it's better if it's suspended above the tank, or secured to a canopy or hood.