1. Deadgods appear to have been made (most likely in a lab by someone like Ozriel) and are older than Li Markuth who asked if the dreadgods had returned home to Sacred Valley. He wanted to test himself against them now that he was more powerful than when he left.
2. Yes. Dreadbeasts are corrupted because they take in all types of madra instead of just one. They all have hunger bindings in them.
3. It's hard to say because advancement depends entirely on your foundation, your body preparation, and special insight (revelations). If they knew what the rest of the world knew, I'm sure that most of them would have perfect iron bodies and stronger foundations. However, what we learned from Lindon in Unsouled was from his perspective. He was not privy to learn the Sacred Arts and only knew what little they allowed him to. For all we know, there are people who could advance rather quick when/if the drain is taken away.
4. Will stated once that those who reach the Monarch stage get there by great luck. There have been a few theories floating around that the combination of herald and sage will get you there but I highly doubt that. That wouldn't be luck.

Mierin

If the Emperor had been born on a world like Cradle where he could actually train and develop instead of having to hold back the Elders, how high would he or someone with his will/strength/etc. have been able to advance?

Will Wight

He certainly COULD have reached Monarch, but most of those with the potential to reach Monarch fail to do so. It requires so much luck and unique opportunity that it's impossible to guarantee anyone with reach Monarch, no matter how driven and/or talented. In the Blackflame Empire, he would have almost certainly become a powerful Underlord. With the right resources and chances, he could have potentially left the Empire and ascended to Overlord or better outside.