leak :
1. always seal up leak from the source. leak from outside, seal from outside, no point seal from inside as seal will fail easily.
2. since it is glass and outdoor, something like silicone weather proof acetic type would do fine.
3. for added durability, cover the seal with aluminium tape or those rubber flex tape.
4. check the cement for cracks/microcracks........especially at the places where the cement makes a right angle. can fill them up with water proof putty filler for big cracks, for small cracks, can use water proof paint.

heat :
1. if want to have light with lesser heat, can consider those car tint like solar guard/v-cool etc.
2. if totally don't want the light to go through but don't want the hassle of removing the glass, can just paint the inner surface of the glass...........then it will look just like ceiling.