Lines 46-55 Summary

Get out the microscope, because we’re going through this poem line-by-line.

And I untightened next the tress
About her neck; her cheek once more
Blushed bright beneath my burning kiss:

Now the speaker unwraps Porphyria's hair from around her neck.
He says that her face is still rosy as he plants a wet one on her cheek.
We can't claim to be experts (fortunately), but our many hours of watching CSI suggest to us that a strangled woman's face would be unattractively blotchy, rather than pretty and rosy. So, either the speaker is lying, or he's totally delusional. It could easily be either.

I propped her head up as before,
Only, this time my shoulder bore
Her head, which droops upon it still:

Now the speaker "prop[s]" Porphyria's head up on his shoulder. This action is a reversal of their positions earlier in the poem, when she moves his head onto her shoulder (check out "Symbolism, Imagery, Wordplay" for more on that reversal).
He's the active one, now, and Porphyria (who's dead, after all) is the passive one.
Line 51 ends with a weird and unexpected word: "still." What does he mean, "still"? Is her head "still" on his shoulder? Like, as he was writing this? This word introduces a whole new level of creepiness.

The smiling rosy little head,
So glad it has its utmost will,
That all it scorned at once is fled,
And I, its love, am gained instead!

The speaker didn't mind assuming things about Porphyria's emotions and desires when she was alive, so why should he stop now?
Porphyria isn't even mentioned by name here: she's just a "smiling rosy little head." She's been reduced to a mere object.
Calling her head "little" is also a way of infantilizing her, or treating her like a child.
He says that Porphyria is "glad" that her "utmost will," or greatest desire, has been attained: everything she "scorned," or didn't like, is gone ("fled"), and she gets to be with her lover! Forever!
Notice that the speaker says "it" instead of "she" in line 54 and "its love" instead of "her love" in line 55. He uses the pronoun "it" to replace "head," treating Porphyria as though isn't even a person anymore. She's an object.
As an object, she can no longer argue with the speaker's interpretations of her desires and emotions. He can project anything he wants onto her, and imagine what he likes. She'll never complain.