1. What if the reseller closes down his shop in the future. Who does the buyer/licensee contact in the future for any support and upgrades for SW?
Contact SolidWorks and your account will be transferred to a different reseller in your area.
2. If the buyer buys a SW license from one reseller, can he upgrade the license from another reseller? For example, if one buys a SW Professional from Reseller A, can he upgrade to SW Simulation Professional from Reseller B?
You will have to request SolidWorks and explain them if you're not satisfied with reseller A and then they can transfer your account to reseller B. Then you can upgrade your license