I found it interesting that this flower is called Devil's paintbrush. It actually resembles the flaming eye the opening image.

Must is an ambiguous enough term to offer multiple interpretations. The obligation or the heat (rutting), which in biological terms is in a way an obligation, an urge whose inhibition may cause great distress. There is also the association with mustiness or dampness, humidity/mold after the heat of ember-like passions, of epic battles? The last imagine may illustrate that state (the mold-invaded ship); many dragons are actually associated with water in some mythologies, so it may not be that paradoxical that fire and water can coexist or even complement.

The poem starts with a question that may very well be a rhetorical one, considering the word "succumb". Should we succumb to being strained? What is it about us that is coarse, that needs separation or purification?
Who determines what is impure?
Then follows a proclamation:

Though I succumb to flawless sieve,
Flow the cosmic coil of loss;
Swim the tribute, earnest give,
Til the sky I emboss.

I like the idea of embossing the sky

We usually think about the sky or heaven in terms of a place we may have access to depending on our good deeds, a place to be inhabited, not carved or molded (multiple meanings again, playing multiple images). Does this suggest that rebelliousness and courage will leave marks not only on earth but also in heaven?
In certainly seems to be the case in legends and myths.

Whether they are bloody sacrifices or not depends on cetain circumstances, but the battles of old continue to be fought with different armors and different weapons, but all subjected to the same devouring power from above that ends up scouring (another ambiguity) all flaws, maybe to punish them, maybe to polish them.

Living and dying can be very simple or very complicated.
What is worth living for? What is worth dying for?
What mosnters do we fight? What monsters do we join?