I'm not sure if I've found The Explanation regarding the questions posted in the OP, but I pulled out my 1987 copy of Mr. Scott's Guide to the Enterprise
and it explains the reactor room on page 88. It seems to mostly agrees with the theories expounded in this thread:
A recent addition to Enterprise's warp engineering section is the dilithium reactor room. This radiation-proof chamber was installed while the ship was in drydock for replacement of her photon torpedo system; it provides unobstructed access to the vessel's dilithium couplings, allowing the crystals to be easily replaced in the event of crystal burnout or other damage.
The dilithium couplings are contained in a pedestal in the center of the room. A transparent double wall, which contains radiation-dampening gases between the two panes, insulates the chamber from the remainder of the engine room. A rotating door, also transparent, provides airlock-like access to the reactor room interior.
Overload of the dilithium crystal couplings may cause a leakage of excessive radiation into the room interior. Should this happen, an automatic radiation-dampening system activates which, after several hours, returns the room to a habitable condition. Only major vessel damage, centered in the engineering section, could cause such catastrophic crystal failure as to produce radiation spillage of fatal proportions. Replacement of crystals or repositioning of the mounting couplings may be done manually by removing the dome of the containment pedestal; however, radsuits (with helmets) must be worn during this procedure.
So, there you have it, from Mr. Scott himself! (Or Shane Johnson, author)