1. If you want to use s:validateAll on other xhtml file you use the s:decorate tag that points to other xhtml file. See the start-up apps that is generated by seam-gen to get the idea of what I mean.
2. And yes you can place the h:message tag in other file and include it from another file (I haven't tried s:message yet).
Use h:messages instead to get messages for the whole view