Thank you for spending time and effort to help improving the game, and here are my opinions on your feedbacks:
1. Mid - bosses: This is an absolutely amazing idea to be implemented in the game and I wish I realized this earlier.
2. Item drop: The reason I didnt implement the health item drop is I think Reigh's healing mechanics is somehow balanced enough if you kite and avoid battle when you are low, but I agree that the value might be a bit low / take too long to reset HP to 50%. However his Ultimate skill is very broken in my opinion as it negates all contact damages and heal up when colliding with enemies. His shield's triggering mechanic has no CD and number limits and it is able to raise your HP from 1 to 100 in a second. However, item drop that temporarily boost to DMG / Ultimate meter / speed would be a good idea.
3 & 4. Character roles & Gameflow: Yeah I agreed to your opinion which I did too carefully or cowardly when comes to allocating stats for their role because the intention is to let players to swap among characters to handle different situation. My initial idea for each of them are: Cedric - excels in single target damage, Bogart - excels at mobility and AOE shredding, Reigh - Defensives and Tanking. Hence, in this case, the intention for them is using Cedric when Cthulhu spawns because he is able to nearly one-shot the boss if his attacks is not split up by other mobs, so to avoid the damage being split onto other mobs, then you can use Bogart to shred them all. Also, if you are overwhelmed by the other mobs and low, you can swap to Reigh has he can regenerate health meanwhile he is able to rapidly knockback enemies in exchange of his range. These are the main reason I watered their stats down so they are going to cover each other's weaknesses.
Anyway I do really appreciate that you did express your opinions clearly and politely ^^