hard is the word we prefer use to describe a fall.
"a heavy fell" "she fell down hard on the ground" "she fell down hard to the ground"
"she took a hard fall to the ground".
For heavy when we use it is like this"
"she took a heavy fall to the ground"
although your sentences are fine we do not usually add the "ily" or "y" suffix/affix
the natural way is to change the position of hard/heavy with ground. Put the words hard/heavy after "down" and leave rest of sentence how it is.
what happened she feel DOWN + (to the ground) how did she fall she fell HARD
The description of how she fell goes before ( + to the ground) if you want to add this.
Final sentence "she fell down HARD ->to the ground"