Capital gain

1. Yes, it will be LTCG over sale of rights in the property.

2. Not possible, against this sale.

Exemption against purchase of any commercial property is not at all eligible under any section.

But against purchase of any residential property in this case is also not possible, as he has more than 1 property at the time of sale of the property rights, and the claim would be u/s. 54F of the act.

Advised to get exemption u/s. 54EC.....ie. investment  NHAI or REC capital gains bonds upto 50 lakhs.

1. Date of agreement will be taken as the date of purchase and in your case, it will be considered as LTCG ( as per latest ITAT judgment). When the sale is likely to take place on December 2019 of the property in which the possession is to be gained will be charged to tax as LTCG. 
2. Exemption u/s 54 can be claimed by investing the sale consideration in another house property, 1 year before or 2 years after or 3 years in case of construction from the date of transfer. 
3. In your case, if the sale is made on Dec 2019, then 1 year before will be Dec 2018, within this period another house property should have been purchased in order to claim exemption u/s 54. Since you have purchased an under constructed property in May 2018 this exemption will not be available. However, exemption u/s 54 can still be claimed if another residential property is purchased within 2 years or 3 years in case of construction within the date of transfer. 
4. As explained above commercial property will not be covered under any exemption provision. Other options are to invest to the extent of Rs. 50 lakhs in bonds covered u/s 54EC. 
please correct me if the above interpretation has an alternative view.