See the thread I linked above. They eventually admitted the fiancé as K1.
I assume that if they weren’t admitted as K1 (let’s say that they couldn’t find the paperwork to satisfy cbp), they could probably adjust by filing I-130 with I-485. That would give them the basis to adjust (immediate relative of a USC), instead of the I-129f.
Again, these are my educated guesses. I’m not an attorney. I’m curious to see if someone knows to confirm or refute my assumptions