The failure of the landlord to fix this problem would likely allow you to to withhold a partial or the entire payment of rent and simply put the funds in a separate escrow account to show that you are acting in good faith. What you are describing is basically a "constructive eviction" by the landlord for breaching the implied warranty of habitability. Your normal options are to move out or offset the rent because you are not getting what you are paying for.
Your landlord can refuse to renegotiate the rent and not extend the lease if he feels that it is too problematic to continue to deal with you. That is his right. You need to mitigate your damages. Yes, you can "possibly" sue for the landlord exposing you to toxins that caused medical bills. However, not moving out of the residence and continuing to remain in there knowing of the toxins and methane gas will be used by the landlord's attorney against you and very well might not be viewed well by the judge.
Relying on the warranty of habitability will require you to show that you provided the landlord with the proper written notices pursuant to the statute. The usual remedy for such a violation is for you to terminate the lease and move out. It does not allow you to withhold rent - particularly if you are not using the rent to make repairs.
You might be able to pursue a personal injury claim against the landlord, but the fact that you have chosen to live with a condition that you knew was dangerous for two years will likely be fatal to your claim.
Your best course of action is to find a healthier place to live and move out as soon as you can.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.