Answers
In order for a property purchase to be used for the Golden Visa application process, it needs to be explicitly included in the purchase contract that it will be used for that reason. Otherwise, it cannot be used.
As the owner of properties in Greece, you have the right to apply for a golden visa if you meet all requirements of the Greek immigration law. One of these requirements is the method of the contract amount's payment. A lawyer will advise you about the procedure and if you are eligible to proceed to the application for a Golden Visa.
You have to consult with a greek real estate lawyer.
According to the legal framework, there are two different time slots for third-country citizens who have purchased real estate property in Greece in the past and are interested in applying for Golden Visas now. The first slot is before Law 4146/2013 came into effect. If the payment submitted before Law 4146/2013 came into effect is smaller than 250,000 euros but the current objective value of the real estate property exceeds or is equal to this amount, a certification by a notary must be included in the documents that are submitted, stating that the objective value of the real estate property as it stands today is equivalent to the amount of 250,000 euros. The second time slot is after Law 4251/2014 came into effect. The submitted acquisition contracts are acceptable under the condition that the existing legal provisions, especially regarding the payment process of the value, have been followed as in force now. The best option is to address your case either to a lawyer of a consultancy.
Not for a golden visa, but you could apply for other residence permits or even citizenship according to your status.
You can only use the properties if you used the Golden Visa-compliant way to transfer the payments. Otherwise, they cannot be used. You need to contact a lawyer.