I would guess that starting the habit is harder wirh adhd. Something doesn't become a habit after just doing it once, you need to do it for a while first. Adhd can make that harder both because you might simply forget and because of executive dysfunction.
I would think that for people without adhd the "forget it once and suddenly it's gone" also applies (seems consistent with my understanding of human memory), but restarting it might be easier so it doesn't feel like it.
Yes. Until they get so extreme they ruin your life. That's the whole thing with disorders. (Almost) everyone has some anxiety. Only some people have it so bad it interferes with daily functioning. Adhd is the same. Everyone procrastinates, forgets stuff, gets distracted. Not everyone is incapable of doing basic shit like taking out trash bags for months.