Thanks for clarifying on the start of the game and who you thought had awkward dialogue. Ronove referring to you as "sir or madam" is supposed to be intentionally weird and is supposed to be emblematic of his total disinterest in accommodating or helping you.
~10 deaths on the Mandrake boss and Orobas is about what I was aiming for difficulty-wise. 20+ is maybe a big too high for Sarcoph Augustus, but then again you said you weren't using the Hourglass, so that would have made things more difficult than it needed to be. So I'm not sure whether I should touch him.
The higher difficulties mostly affect the density and speed of projectiles, and well as the resting time between volleys/patterns of bullets. The seedling minions move at the same speed on both Easy and Intended, but there are more of them on Intended, so that would create the effect making their pursuit feel more threatening.
In terms of being "forced into a corner" during boss battles, I agree with you. That's been something I've been trying to design bosses to avoid. I think the Mandrake's design makes it so that it's pretty optimal to just stay back in the corner and move yourself just enough to dodge bullets. However, Sarcoph Augustus and Orobas were both designed to make it so the player should have to utilize most of the arena to be successful.