These verses do speak of believers.
Paul is worried not for the loss of their salvation but for their loss of abiding in Christ and the loss of the experience of living under grace... not losing the position of grace ... which is not possible.
Paul is worried, that for some of those among this church would not gain from all his work in hopes to share with them the truth.
This is for all of these people, those who are in the church and believe to be saved as well as those who truly are saved.
If the preacher at local congregation is sharing the gospel constantly, I am sure he would share these same fears... ( gal 4)
gal 5 4-5 is again a comparison between those who would think could "help" justify themselves to those who know that the righteousness of Christ is what justifies us...
These verses address the church as a whole
not just believers or unbelievers but the wheat and tares
"fallen from grace" as to say having heard the truth but muddied it with "adding to" via laws/works
and a " little leaven leaveneth the whole lump" goes to show how even adding a little bit changes the whole gospel...
pretty sure all of us agree on that right?
Eg was pointing specifically at "those who Christ became of none effect to,those who do not obey the truth"
these would be those with the added "leaven"