[NullAshton]

Romulan cloaks don't work by masking emissions, they work by phasing a ship a bit out of tune with reality. In theory, a perfect cloak would be indetectable, and the cloaked ship would be able to move through other objects. The energy is expended in remaining out-of-phase. My guess is that the expended energy is re-emitted as tachyons, like heat is emitted during most reactions.

ST reactors do not 'absourb' gamma rays, rather, they use the dilithium crystal to channel the energy into a purer form, called 'plasma', which is redirected through the plasma conduits to power the main systems, and isn't very regulated. Fusion power is used for the items that require less power, such as lights, or those touch panels.

Exhaust could be contained... but for regular space-travel it's probally so small as to not give away the ship. They just didn't think that exhaust would give away a cloaked ship, since sensors probally weren't programmed to detect it...