GMAT Verbal & Essays

Starfish, with anywhere from five to eight arms, have a strong regenerative ability, and if one arm is lost it quickly replaces it, sometimes by the animal overcompensating and growing an extra one or two.
1. one arm is lost it quickly replaces it, sometimes by the animal overcompensating and
2. one arm is lost it is quickly replaced, with the animal sometimes overcompensating and
3. they lose one arm they quickly replace it, sometimes by the animal overcompensating,
4. they lose one arm they are quickly replaced, with the animal sometimes overcompensating,
5. they lose one arm it is quickly replaced, sometimes with the animal overcompensating

Here, in the correct option if we remove the part “and if one arm is lost it is quickly replaced” the sentence wouldn’t make sense right?
Since this would be left “Starfish, with anywhere from five to eight arms, have a strong regenerative ability sometimes by the animal overcompensating and growing an extra one or two.
Additionally, isn’t the animal overcompensating “by “growing an extra one or two and not as a separate action?