I think the motive for killing may be everything you posted. I'd add two.More or less, this is just how I see it having gone down. I am VERY much in line with this. Couple notes-
The downed trees along the ground clearly had burns in he CS photos, but they were snow-covered upon my visit to the site. But i did observe the VERICAL trees which were reportedly burned and photographed. I saw a few dark round marks which had been suspected as burns by other folks here, and me as well. But these marks were just mold/mildew spots and not burns. So the fire appears to have been confined to the ground area.
Issues I've got in the theory: I don't see the phone being turned off and back on. I've never accidentally shut off or turned on my iPhone in my pocket. I think it was only powered off once. And for the exact reasons you mentioned numerous times in your theory-he was rushed and panicked. He was fighting with her when he pocketed the phone. Turning off a smart phone can take a few full seconds of depressing a button and then swiping "turn off" (with iPhone) or depressing a button for 1 second and the. Selecting the "turn off" button on an android. Both take some level of attention that he wasn't about to give in the middle of a violent struggle.
On his decision to kill her, which he made at some point, what was the motive?
Being very concerned with his DNA, it would seem his motive for killing may have been that he didn't want her to identify him later. And how would she have done so?
He lives close and could be spotted in the area again
Or
she recognizes him
Or
she saw his car and maybe he knows she might have gotten the license plate number
Or
?????
If he was random person from far away land it would seem he could have left her in woods alive and disappeared. Just a thought.
He planned on killing her in advanced, or he got carried away with the strangling (if he did) because that's satisfying to him.
I think the DNA cover up came after, and that was so either...
He was afraid his DNA would match in the system, (which we now know it isn't in the system)
He is concerned about future arrests and his DNA being identified then.
If it's a random person from far away, That would eliminate him having to know her schedule, but then we are back to square one with...
How did he know about the path?
Why not consider another woman in a more remote area, taking less chances?
As you say, why not leave her alive?