Your question is not clear.
Do you have setup the slave site to have the "/templates" directory partially share with the action = "special copy" + the "template folder" = {deploy_dir}/templates
OR just the action = "special copy".
When you partially share the "/templates" directory, this create a "/templates" directory in which the subdirectories are symbolic links as shown in tutorial video 0.b - slide 34.
www.jms2win.com/en/tutorial#jms12x-0b
1) If you did the setup as I have mentioned above (partially shared the /templates directory) then this is NOT related to open and resave the slave site definition to have the "html" and css file updated.
The only case where this might be required is when you add new templates subdirectory because you install a new templates but not when you update a CSS or an HTML file.
2) You have to open and resave a slave site (manage site) when you want that JMS restore or install new things that was not present.
This can be either extensions or files and folders.
3) Use the JMS tool and create group of slave site as illustrated in the tutorial video 0.b slide 24