Hi Angel @Angelwheel ,
Vertex Snapping shouldn’t affect the Fill (normally), but I did notice that your Selection seems to have been wrong; this Vertex shouldn’t be Selected:

That’s at least part of what created the crazy mess, when you used F the second time…
After Merging, and with the right Selection, the area should Fill correctly in one go, but it doesn’t harm to cut it up in some smaller areas, like Omar suggested.