Yes. You will own your lot in Colonial San Martin free and clear with full title. This is the case in general for all real estate in Nicaragua.
There is a notable exception though. Most beach front property (as in, on the sand) will be a 99 year lease from the government. Again, this does not apply to Colonial San Martin.
Extremely safe. As mentioned previously, it's the safest country in the CAPDR (Central America, Panama, Dominican Republic)
Yes. The area is about 2 hours from the capital Managua and about 45 minutes from Rivas. The villages are filled with expats and the locals are friendly and hard working.
It's hard to find any place better than Nicaragua to retire. Of course it all depends on what you're looking for after retirement. The government has many programs for retiree's to immigrate to Nicaragua easily and in most cases, tax free.
The Costa Esmeralda international airport is about a 30 minute drive away. It was designed to service the Emerald Coast where Colonial San Martin happily sits.

The Managua airport is about a 2 hour drive.