This takes place many years after the original drawing.

The far end of the Dragon Lands is rather cold, as it doesn’t have many lava sources. Dethroned Dragon Lords were exiled here.

From ancient times the Gauntlet of Fire trials existed, used to pass on the Dragon Lord status when the current one has to step down. Most of the times when they had to go into their hundreds of years of hibernation. Dragons can’t have a long time without their Lord. However, this didn’t stopped dragons to take away the status by force. This is what young, foolish Gaius did and Scales did. Scales was also dethroned by force, possible by a young Torch. One dragon can by the Lord for hundreds, if not thousends of years.

Gaius cause of death is unknown. It may be because of some form of illness.

Dragon eggs can be kept ‘alive’ and hatched hundreds of years later, as long as some level of warmth is getting to them. After that warmth is gone they can last for a few months. Gaius used his body heat to warm the egg until he passed. Gaius was unable to hatch his egg, due to his flames weren’t hot enough for it.

While dragons are communal in the way they hatch and for the first few years raise their offsprings together, in ancient times if a Dragon Lord were exiled and they had an egg, they had to took it with them, even it means they can’t hatch them. The weak lord only has weak hatchlings, which dragons don’t need any of.

The massage on the wall was written in old ponish. Celestia taught it to him.

Celestia tried to hatch the egg with her magic, but it didn’t worked. It was alive, but refused to hatch. Celestia feared what if the young one imprint on her (Gaius told her about it) , so she wasn’t sure if the egg sensed it somehow. She kept the egg in “hibernation” by giving it warmth with her magic for hundreds of years, during it trying to find somepony powerful enough to hatch it. When she formed the School for Gifted Unicorns, she put the egg in as a final test. Failing to hatch it wouldn’t have consequences, as it was also used to see the power level of the young unicorn (how much their power has an effect on the egg). She was hoping this is where she finds somepony powerful. Of course, hoping there wouldn’t be imprinting, the young dragon would have been raised by somepony else, as the student are just foals. Then young Twilight came along who managed to hatch the egg, and the later named Spike imprinted on her.

Spike’s mother’s whereabouts and identity is unknown. Celestia did not know Gaius had an egg let alone a partner. (Side note: I don’t have concrete plans with Spike’s mother for now. But I have design ideas, and I can tell you Spike looks a lot more like her.)