A land contract is a contract between seller and buyer of property. A contract is only made when an agreement between seller and buyer has been reached. The seller becomes the land owner only when the full payment has been made.

A seller can charge whatever interest they wish on a land contract. The buyer doesn't have to sign a contract if they don't agree with the terms.

If one of the fee owners didn't sign the contract then you don't have a valid contract. In order for the contract to purchase land and build on it to be valid and binding on the parties it must be signed by all of the purchasers and all of the fee owners.

No

A land contract is just like any other contract, complete with contract law principles. No one can sell the property until either the contract is either fufilled or breached. If it is fufilled, then it becomes the property of the one purchasing the property and after they have successfully completed the contract, they then are free to sell it to whomever they wish. But, if the contract is breached, it is then still the original owners and they can do as they wish, keep the property, sell the property or find another individual willing to enter into another land lease contract.

So, while the property is tied up in a land lease contract, no one can sell or buy it until either a satisfaction or breach of the contract is committed.