Yes, a patent specification can be amended. However, the amendments should not go beyond the scope of the invention disclosed in the specification.
Yes, an inventor or his assignee can file an application for grant of patent in different countries. The applicant can either opt for convention route or PCT route to reach different jurisdictions for the patent.
No, there is nothing called global patent. The applicant has to approach the respective patent office to get a patent granted in his name for the invention.
A patent is valid for 20 years from the date of filing of the application for patent. Once the patent is granted, the owner of the patent is required to pay annuity to keep the patent in force.