How are intervening rights (after the 6 mo. grace period) established if we are pursuing business activities using an expired patent (expired for nonpayment of maintenance fees) assuming the patent holder may attempt to renew or revive the patent we are using in our business? What do we have to do to get these rights? If we get these rights and the patent holder revives the patent do we have rights to use the patent for our business forever?
IANAL, but my take: you don't need to proactively "get" the rights; you would only establish them in court if sued for infringement. Instead, collect evidence/proof of the practice or substantial preparation to practice during the period of expiration. Although ~15 years old, see here:
Absolute rights are extended to those who actually practiced the patent during the period of abandonment, while equitable rights "may" be extended to those who made "substantial preparation" to do so. These rights are limited to the period beginning after the six month grace period after the maintenance fee due date up to the date that the Commissioner actually accepts the delayed maintenance fee and grants the petition.
If you are granted intervening rights ('guaranteed' if practiced, 'maybe' if only prepared to practice), you may continue to practice after revival.
Additionally, note the current "unintentional" standard for revival of a patent expired due to failure to pay maintenance fees, with no time limit for petition.